The speedometer drive seal in the MGA gearbox is quite small and has some rather odd dimensions.
Shaft diameter = 5/16" (7.94-mm)
Housing bore = 9/16" (14.28-mm)
Width = 11/32" (8.73mm)
(1/4" or 6.35-mm width will work)

I have so far not been able to find a manufacturer's part number from the size dimensions. Someone obviously makes the things, so maybe we can find a maker's name on the part.

Original BMC part no:   2A3254

Direct Cross Interchange: ahead4healeys.co.uk GS4921, YGX6135 autosuppliers.co.uk AS10049 BASTUCK G37AH BMC A3254 BMC AAU2304, AAU2304A, AAU2304J, AS10049 BMC C32592JX BMC E401FA BMC GBC265, GBS265 BMC NKC105A (alternates) 108757, 13H1956, 528747 (alternates) AS.10049, C32592, C32592J, S.1405 historic-cars.net G37AH Moss Motors USA 461-445 oldtimer-zentrum.com AHGBC265 oldtimer-zentrum.com AHGBS265 Tokez 850044

Applications: Austin A30, A55 Mk 2, A60, A90, A105, A110, 1800 Austin Atlantic, Cambridge, Devon, Hereford Austin Somerset, Westminster Austin Healey 100, 100-6, 3000,BN4-BT7 non-OD, BJ8 Farina Magnette Jaguar E-Type all - speedo pinion and angle drive at gearbox Jaguar XJ6, XJ12, XJS Automatic and MGA all and angle drive at gearbox MGB 1962-1967 non-OD MGB 1968-1980 4-synchro and LH overdrive MG Magnette Mk-3, MK-4 MG Midget, AH Sprite 948, 1098, 1275, 1500 Metropolitan 1500 with 9/41 axle Mini/Metro Morris Minor 803, 948, 1098 Morris Oxford 5, 6 Riley 4/68, 4/72 Rootes Group Hilman, Commer, Humber, Singer Sunbeam Triumph 2000/2500/Pi Triumph Hearld 1959-71 Triumph Spitfire 1962-74 (3 rail), 1975-80, and 1974-80 J-type OD Triumph GT6 1966-74 Triumph Stag Triumph TR2 1953-55 Triumph TR3A & TR3B 1957-62 Triumph TR4 1961-65 Triumph TR6, TR250(TR5) 1968-76, and A-Type OD, and 1973-76 TR6 J-Type OD Triumph TR7, TR8 5-Speed Vitesse 1962-71 Wolseley 15/60, 16/60

Here we have one slightly narrower that will work.
.312 x .564 x .250ADL - NBR
5/16" ID x 9/16" OD x 1/4" Wide
Available in Type-A (single lip) or Type-ADL (Double Lip).
"NBR" means Nitrile Butadiene Rubber
(good for oil at normal engine temperatures)

The number in bold above is a generic description, which may be prefixed by various different manufacturers' designations.

Supplier: Part Number Global OS.312-.564-.250TC Harwal .312 x .564 x .250ADL - NBR

Now here is one I recently bought from Moss Motors USA (March 2019), showing both sides. This one is just over 1/4" wide, which itself is not important. At first I thought it had 3 or 4 seal ridges, but the more I look at it the more it looked like a right handed helical thread. So I picked up two pairs of cheater glasses and a bright spot flashlight for a closer look. Sure enough, it is a helical rubber seal, about 3-1/2 clockwise turns in a quarter inch width, or about 14 threads per inch. Not sharp crested like a mechanical screw thread, but rounded and blunt edges that look like it would be a good long wearing seal for a smooth rotating shaft.
Both the gearbox and engine cable drive spindles turn clockwise (looking at them from outside), and clockwise rotation of the spindle would auger oil back inside as it turns. However, the gearbox seal is below oil level, so a thread seal may leak oil out as it turns in reverse, or even when sitting still when the car is parked. Now thinking about it, I don't know what kind of speedometer shaft seal is in my gearbox, but will see if I can remember to check to see if it leaks next time I'm under there.