Janet...I disagree with Jena's conclusion; let me explain my thinking on asking you to ask Jena about the original mount.
It is known that after the war, the russkies were cannibalising all contax equipments to come out with cameras that could be sold in the west -ready source of dollar for them.
The serial number of the lens points to 1949 as year of manufacture, the year the first M42 mount camera was released by Zeiss East Germany (the ruskies were telling them what to do, really). Given the shortage of of material, capable designer ( many of the contax engineers came over to W.Germany); and the speed of introduction of a new mount, I thought it might be possible that Jena simply retrofitted an existing lens ( Contax RF design, pre-war design, not to be confused with the post war 85mm which was designed from the scratch in W.Germany.) to a new mount.
My suspicions got deeper when this lens had no reference to any of the M42 listing.
My theory is that they retrofitted some Contax RF lens, possible left over from the war time with the new fangled M42 mount, since they would have wanted to avoid complete retololing before they knew how the new mount will be received. ( There were very few SLRs at that time, and the Japanese were yet to arrive). When the mount was accepted by the market, they designed a new lens tailormade for the system, and withdrew the Sonnar at 85 mm. .
I doubt if anyone would convert a Sonnar RF lens to M42 mount in the early 50s ( your friends father got it from an Army photogrpaher in the early 50s
, especially when the RF lens design and SLR lens design are different. I further believe that to convert a Rf lens to a SLR lens, not only the mount but also the rear element(s) of the lens have to be changed ( retrofocussed) so that the mirror is not fouled ( may be the focusing helical needs changing as well). This cant be done on a DIY basis.
It is possible that this lens does not perform as well as its RF cousin due to the retro-fit design.
So in conclusion:
1. This lens must have been made by Jena on M42 mount
2. This lens would have existed for a very short period; and
3. This lens will therefore be a collectors delight
Ujjwal
P.S : BTW, it has nothing to do with Pentax. pentax adopted m42 much later; though they made it an industry standard.