[monochrome]

The problem I see here is, if I understand you correctly, that the lenses are not entirely the same.
The main problem is that the M42 lenses have a smaller diameter, so it is not so easy to add a K-mount to such a lens.

Below is a comparison of the SMC 105/2.8 M42 lens with the same lens in K-mount:

The lens BARRELS are identical, as is the formula. The coatings and focus rings only were updated for the K version. Even the lens caps on the K-mount are metal dinner plates, same as every Takumar lens.

These lenses were designed with a detachable part at the end - a mount tube. The mount tube is a piece of anodized metal maybe 2.5 or 3 inches long that is removable. It attaches directly behind the aperture ring and was intended to be transferred from one long lens to another as an economizing measure. It was then a simple matter to change only the mount tube from M42 to K-mount. You could update a Takumar 500/4.5 to K-mount (SMC Pentax 1:4.5/500) by simply buying a replacement tube - but they are all gone.

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Last edited by monochrome; 01-02-2016 at 02:32 PM.

[pathdoc]

The lens BARRELS are identical, as is the formula. The coatings and focus rings only were updated for the K version. Even the lens caps on the K-mount are metal dinner plates, same as every Takumar lens.

These lenses were designed with a detachable part at the end - a mount tube. The mount tube is a piece of anodized metal maybe 2.5 or 3 inches long that is removable. It attaches directly behind the aperture ring and was intended to be transferred from one long lens to another as an economizing measure. It was then a simple matter to change only the mount tube from M42 to K-mount. You could update a Takumar 500/4.5 to K-mount (SMC Pentax 1:4.5/500) by simply buying a replacement tube - but they are all gone.

Looks like Pentax took some hints from Tamron...

[tim60]

More like they were planning transition to K for a long time, or it was a manufacturing and support efficiency measure.

Which lenses have this common part?

[blende8]

I have three lenses where I can make a direct comparison:
A 105/2.8
B 120/2.8
C 135/2.5

I measured directly behind the aperture ring:

Takumars:
A 5,60 cm
B 5,60 cm
C 6,50 cm

K-mount:
A 5,85 cm
B 5,85 cm
C 6,60 cm

Measured with a caliper.