Pentax K body + M39 lens
Unfortunately the flange focal depth doesn't allow conversion.
There was an item titled M/L39 which was actually an alternate name for the L39 with a rangefinder. Just to make life more complicated, the Soviets used the M39 designation not only for a rangefinder, but an SLR system as well, which is a 39mm wide threaded connector with a flange focal depth of 45.2mm. This is very similar to the M42 and several early Helios optics (for example the Helios 44 2/58) were available both in M42 and M39 (SLR) version.
Nowadays most misunderstandings besides the names stem from the fact that there are M39-M42 adapters sold on eBay, which are actually rings made of very thin material and threaded on their in- and outside. Screwing them on a 39mm thread they convert it to 42 mm wide allowing to fix an 39 lens on an M42 mount. However, you might have noticed that the flange focal depth of the M39 is less than of the M42 (45.2mm vs. 45.46mm, 0.26mm difference).
The difference might seem to be marginal, but anyone who is familiar with manual lenses and had faced the issue with focusing on distant objects will know this minusucule difference will be decisive when very trying to sharpen a distant subject's image.
The usual solution - to buy a simple M42 adapter and an M39-M42 ring, very carefully sand off 0.26mm won't work here due to the similar flange focal depth
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