I do not know the answer to your first question. I can only assume it was a common back used for another 24XX caliber watch in a similar case.I wanted to ask two doubts:
1º.- I tried to look similar to my Stolichnie caseback, and I have not found it. Out of curiosity, someone has in its collection a similar caseback, or know what could be the origin of the caseback, in which factory was produced ??
2º.- I have seen that the catalog of 1960 only refers to two models of Early Stolichnie 16 jewels dials: one with my dial and the other with the Mroatman´s dial (very nice and similar to a Signal 2612 dial). I have not seen other examples of my dial Stolichnie, which gives me a little scared, because my dial, but has obvious signs of wear and time, does not indicate "СДЕЛАНО В СССР" or "made in USSR", or may have disappeared. It may be due to the condition of the dial, but do you believe it´s a genuine dial?
As for your second question, you don't need to worry, your dial is absolutely authentic. I think somebody has scrubbed away the lume, minute track, and "Сделано в СССР" inscription in what was perhaps an overly-aggressive cleaning attempt. The perimeter of the dial is most susceptible to dust, dirt, and staining, so maybe a previous owner just wanted to clean up the dial a bit. I will include a photo of my own watch (not yet on my website) with the same exact dial and movement.
That said, the "Сделано в СССР" inscription is not required for a dial to be authentic. See below for two watches in my collection, distinguished only by the printing on the dial. Both are entirely authentic, in my opinion.
I assume the presence or absence of this inscription could be used to date some watches, but I don't know which came first.