Returning to hobby after some time away, like 15 years !

What is the difference between RED 3502 and RED 3501 if any. ?

Is the 2-pack, 3509 ,  RED 3501 ? The tubes are not labeled.

Is the original RED 3502 still available ? 

Years back I built Race Cars ,  Muscle  Cars , some Military and Corvettes, semi PRO I would say.

What am I working on now ? Big fan of Military and have a rather large collection of kits from  years back.

I've started work on  this earlier issue Testors Kit.  I have Testors 3501 which appears to be ok.

Many thanks in advance.