I guess the discussion here is not what it is an A or B version. As you say, the effects loop configuration makes the difference.
The main question here seems to be the year in which the change was made. It happened in 1993, although some people at Mesa were saying it happened in 1995.
The secondary question here is when Mesa made the transition between the "early" B and the "full" B (as Boogiebabies called it).
That s/n 6038 is an early B (Feb 93), as well as mine (according to Mike B.)
Boogiebabies asked me some time ago some specific questions about the components or the preamp tubes on my amp and, as I can read here below, he has concluded that his s/n 689x (Dec 93) is a full B but with the power cord attached.
Thus, both transitions (A to early B, and early B to full B) happened in 1993.
I am interested in what some people call the transition between "early A" to "late A". Is there any difference or is it just fiction?
Some people call the "early B" as a "no A/ no B", so it could be an indication that there were only 3 "different stripes" in the Mark IV development.
Regards