Question about screw-mounts

I have just bough an M42-PK adapter from Percy on this forum, and it came with another, larger, screw mount adapter which screws into the M42 adapter. This evidently provides mounting for lenses with a larger screw-mount than M42. This adapter also has it's threads on the outside, so is obviously for lenses that have an internal thread. Does anyone have any idea what this mount could be?

Finally found what it is on the internet (I think): it's a T-2 mount, to fit some lenses made by Tamron in the 60's. I'll have to see if I can find any cheap ones to try them out.

The first version was just a T mount then they brought
out the T-2, they are both usually interchangeable.
Quite a lot of pre-set lenses used them including
Russian made telephotos. So a Jupiter 11A 135mm M42 can
be converted to a K mount by swapping the T mount.

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Not sure I quite understand? You mean the T mounts on the lenses were removable?

If you look on Wiki.. about T mounts you will find out the specs.

A lot of old preset manual lens had a T Mount. This helped the stockists as they only had to stock the basic lens and then could sell the T mount Adapter to fit the various makes of camera.

Some Spotting scopes have this Screw thread on the top of the Eyepiece which means with a T mount/ To P K adapter and cheap extention tubes, (needed to get the correct focus), you can get a really long lens to fit your pentax.

Be careful with the Two different threads as they are easily stripped if fitted in the wrong place. Pentax/ Praktica M42 is 1mm pitch and T2 mount is 0.75 pitch.

Patty