Ah but 78 is 78 

The two speed 78.26 rpm and 77.92 rpm are dependent on the strobe being 
calculated on 60Hz or 50Hz frequency. I would just stick to the pitch and forget 
the speed altogether as there are too many variables – too much drain on the 
supply will drop the frequency in any case, lots of such arcana to contend with. 

Not that you can be sure that it was A=440hz, it may have been 435 or 443 or 
just about anything on the day of recording. 

Best thing is to stick to a frequency typical of the time and place of the recording. 
If you don't know what that was – I mean who does know – I would first try it at 
A=440Hz and see what it sounds like. Maybe ask a few other people to have a 
listen as you might have to lower the pitch a bit, even the opposite and make it a 
bit sharper.  After a while you may get to feel your way around the problem 
through listening. 

Just remember 78 an expression of hope rather than a statement of fact.