How can I tell if a MKIV is the a or b revision

[Boogiebabies]

A serial number past 6200 is likely a B. A detached power cord is also a B.

 

[phyrexia]

I think the tranny is in all of them, no need to worry bout that.

 

[Mongo1]

Here's one way to tell.
MKIVa has two mono EFX loops (normal & switchable)
MKIVb has one loop (single send with stereo returns)

MKIVa

MKIVb

 

[Boogiebabies]

I think the tranny is in all of them, no need to worry bout that.

Many A's have a 562004R1 OT as well as many early B's. Shortly after mid 1994 the OT was replaced with a thicker 562004 more like the oler MK III 562003.