Generally speaking, there are two types of legacy lenses... ones that fit old rangefinder cameras, or the ones that fit old SLR cameras. The distance between the
lens mount to the
back of the camera body is shorter in rangefinder cameras because there is no mirror.
Usually lenses with M39 threaded mounts are rangefinder lenses, and M42 threaded mounts are SLR lenses -
BUT, not always.
It seems that the adapter you need for your Industar-50 is the longer M42 adapter (but you need a step-up threaded sleeve that will change your lens from M39 to M42 mount. Another way (better way IMHO) of doing it is to add a "spacer tube" in between your M39 adapter and your Industar-50.
You can buy an "M39 extension tube" sets, they are not expensive. They usually come in several tube lengths (all M39 thread) which can be joined all together or separated individually. Make sure the set you buy has one that is 16mm long, I think this is what you need so that your Industar-50 will be able to focus to infinity (this is what worked on my M39 Mir-1). The link below shows a set with extension ring lengths of 26mm, 16mm, 8mm, 5mm.
4 EXTENSION TUBES tube Step-up rings ring for M39 Leica FED mount camera 930 | eBay
The downside is that your pancake lens, won't be so compact anymore. The bonus is that you can add the other unused tubes later to take macro shots with any of your M39 lenses.