25D Dizzy, which one?

["RossOvercash" <jroverca@tiac.net>]

OK the 25D dizzy was used through the 1974 model year.  There were a number
of different model 25D's produced.  What is the difference between them?
How do they effect the operation of the engine?  I'm just curious because
the spring in my vacuum advance is broken and the choice is a replacement
advance or the possibility of a used 25D of a different model number!

Safety Fast!!

Ross Overcash, 74B, NAMGBR 2-1172, Ayer, MA
http://www.tiac.net/users/jroverca/index.htm