Both made by Kiekhaefer (Mercury)
WM7 uses Mark 20 powerhead and KG7 lower unit
WM7A uses Mark 25 powerhead and KG7 lower unit
More detailed information would require you to
compare parts manuals for Mark 20 and Mark 25 powerheads. I believe both parts manuals are on
Kiekhaefer had to walk a thin line between
a large volume customer ( Western Auto) and the ever
unhappy Mercury Distributors and Dealers. Wizards
were seen by them as unfair lower price competition.
Kiekhaefer valued the economies of scale that Western
Auto's large orders made possible. Excess inventory of prior model Mercury parts assembled into Wizard brand outboards was probably found money to Kiekhaefer.