Afternoon Roy
I may be able to offer some help regarding the numbers that you listed
The Hepolite info that I am looking at is thier 1939 catalogue.
The highest piston ref number is in the 8000s therefore I guess the 9747 and 9749 may we'll be post war?
Ref 4870 is for the 1932 K32 350cc OHV machine. It is an aluminium piston (not heplex) of 60.8mm dia with three rings and has a flat head.
Ref 1600 Is for the 1926/9 350sv models B28, B29, EW and is the aluminium piston type (not heplex) of 60.8mm with two rings and a domed head. (NOTE 1602 is the cast iron type if you should have them)
Ref 5788 is a bit of an enigma! It is listed for the 1935 750cc OHV special?? Again this is an aluminium
(Not heplex) piston it is 76mm dia with one compression ring one scraper ring and one oil ring that is below the pin and a flat head. I don't know the machine that this is for.
I do have some other info for later machines but currently cannot find it.
Let me know if I can be of further help.
Steve