Very simply...

aaaa-bc.bas

where 'aaaa' is the Project Code
'b' is the PCB Revision
'c' is the Software Revision

example...

9912-2F.bas - Project 9912, PCB Version 2, Software Version F.

The software ending with the highest letter of the alphabet is always the one that is used for current production burn.

Remember to document your changes in comments in your program header. In six months time you'd have forgotten what you did and why.