Morris Minor Owners Club

I've got 4 gearboxes - two are 1275 Midget boxes, two are 1098 Minor boxes. The serial numbers stamped on the casings are:
1275 Midget MB 1527, and MB 39010 L
1098 Minor M 28737, and MA 19 708 9

I assume the B on the Midget boxes signifies the later B type gears, but wondered if anyone knows what the other numbers and letters mean, I.e. the A in the other Minor gearbox number, and the L on the end of the Midget gearbox number? All these numbers are stamped in the same place on each casing using the same style of punch, so not remanufacture references ( which are stamped elsewhere on the rear of the casings).

Not sure, but it seems many standard Minor ribcase boxes have MA..... stamps, perhaps standing for Morris/Austin, seeing as they were also fitted to Austin A35s, Austin A40 Farinas and Austin Minor vans?

I currently have 5 gearboxes (in various states of completeness!), and they have the following numbers:

3X standard Minor 1098cc ribcase boxes;
MA 86089 (currently being removed from the car as it crunches when dropping down into 3rd.)
MA 152381 (possibly the car's original gearbox, in need of rebuild.)
MA 308620 (I rebuilt this a few years ago, intending to fit but never got around to it.)

1X MG Midget close-ratio ribcase box;
J996 (intended replacement for the one that's in the car at the moment.)

1X believed to be Austin-Healey Sprite ribcase box;
A8H248 (Another one that's in need of rebuild.)

I'd be interested to know if anyone has further info on the gearbox coding...