I don't believe the original firmware versions built into the cameras have been released as they are not updates. The closest would seem to be v1.01, Ricoh have a zip file of it here:
http://s1.media.ricoh-imaging.co.uk/media/2a477a0ca30d88b15ce6b22f97d5edfc/Firmw...
or see this page:
http://www.ricoh-imaging.co.uk/en/Photo-Downloads.html
you might need to check with Ricoh if that is the closest to what you want or perhaps ask them for the v1.00 file. You never know.
At least those are from the manufacturer and not hacked files where you can never be sure quite what you're getting!
Not sure whether downgrading would cause any problems though, it's certainly a do at your own risk thing.
This site:- http://www.pentax-hack.info also has some earlier firmware downloads but it's a third party site so even more at your own risk. The earliest they look to have available is v1.10.
I've often wondered why people would want to do that as you'd loose functionality.

Edit: I've just downloaded Ricoh's v1.01 update and it's actually v1.10, so the sensible option would seem to be to ask Ricoh directly for v1.00. Alternatively, ask the Pentax-hack site if they happen to have it.

This page may be of interest too:
http://ricehigh.blogspot.com/2007/09/how-to-get-back-to-older-version-of.html

What do you consider to be a fair price?

The DA* 300mm is officially designated FF by Pentax (as is the DA* 200mm too.)

Yes, it's just one photo, so one actuation. I can appreciate that regular continuous shooting pushes up the count more rapidly though!

My intention was to suggest the alternatives, of which David's was but one!

It won't if it's one of the new HD ones as the aperture ring has been abolished for some strange reason.
While the aperture can be set from the camera if the tubes have the electrical contacts, it can't be set with tubes where these are not present.
Ricoh weren't thinking straight when they redesigned this lens.
It may be worth obtaining an old manual setting macro lens or the older style Pentax one which has an aperture ring, or of course, extension tubes which have the electrical contacts.

Yes, no problem with that. But it's good to know you're starting off point before adjusting it. On a bright sunny day it isn't always easy to see what's on the screen after taking the shot (and it might be too late to repeat it.) If one lens you're using gives you what you expect and the other doesn't, it is a recipe for getting 'off' exposures. The thing is though, the camera is reporting what can be considered to be correct exposure but the lens isn't delivering that! Swapping lenses will lead to unintended results.
I know it isn't a huge difference but it is a difference non the less.

The lenses used in the linked article are not the current FF DFA lenses so on the basis my APSC DFA's are fine too, that system will work as the photographer appreciates a reliable starting point.

Yes, the camera determines the exposure setting. The body should then should deliver the correct speed, the lens should set the correct aperture.
In my comparison shots the camera determined the exposure was identical for both of them. But the delivery was not the same. It seems the aperture requires further stopping down to what it is actually doing. If the light transmission is different, it should be allowed for in the electronics in the lens.

If only there was a little accessible potentiometer on the lens one could tweak to adjust it!

Thanks John. I'll see what Ricoh and SRS say. It shouldn't be necessary to have to tweak every image in Photoshop due to consistent over exposure, though I realise one can! When in a hurry it is easy to forget the lens really requires dialling in under exposure each time. Had it been half a stop under exposure it wouldn't be so much of an issue (though it might if used for negative film.)
All I can say about it is that when I first tried the lens after the lens arrived it consistently burnt out the skies whereas my other lenses didn't, or nothing like as much. Couple that with most of the shots I've seen taken with one on the K-1 in sunny conditions published on the web also look over exposed!

If the batch of lenses has had a manufacturing defect or an incorrect electrical setting issue Ricoh need to put the matter right, so its best to try and sort it out while the lens is still new rather than regret not doing so at a later date. Perhaps Ricoh are not even aware of this, though you'd think they would be.
Even an old F 28-80 that came with a film body for £5 at a boot sale gives me better exposures on the K-1, just a shame it's bad for chromatic aberration! I'm surprised you didn't pick up on this in the review of it, though if you always dial in -0.3EV as per your comment earlier you've possibly got the exposure closer to what it should be. But you shouldn't normally have to do that as the starting point.

When one sets the camera settings memory to return EV compensation to default (zero) each time the camera is turned off it is easy after getting the first shot right to forget to adjust it for subsequent images. In any event, this isn't a budget brand X lens after all!

They both meter the same way Algi and the difference is consistent. If there was an adjustment to tun the 28-105 down half a stop I'd do so. It's metering the scene OK (as both lenses gave the same reading) but not translating that reading through to set the f stop correctly. It seems it isn't closing the lens quite enough. I do worry about it as when cheaper optics perform better one wonders if it is worth purchasing the newest and supposedly the best. My Sigma 24-60 gives good exposures as does my FA-J 18-35 using exactly the same exposure method.
It's just the 28-105 that consistently over exposes. On that scene it gets away with it, just. But on more contrasty scenes it doesn't and burns out the highlights. I can lighten an image in PP if need be, but darkening a burned out one one can make things look rather awry.
Why buy the latest and supposedly the best if older more affordable lenses perform the task better?