The numbering system, while not always exact, seems to indicate 1500CC (or, 1489, anyway), the A meaning Austin, U meaning central gear change, L meaning low compression, not sure on the last A.
15G would be MG, 1489CC, G indicates MG.
15GE is a Magnette number, as are others, however, someone said somewhere they estimate over 60% have been "uprated" to more modern and powerful MG engines by owners.
That said, I've seen Wolseley numbers like 15AMW, which by "code" should have had the "W" for Wolseley behind the 15, yet it's naught.
I have enough trouble putting my sox on the correct feet.