'500 I am thinking your A7 is an early S/A model with half width drum/sprocket and QD rear wheel.
The bearing in the brake drum is an imperial size, the same size as the wheel bearings. Size 7/8" x 2" x 9/16"
Looks like you have ordered the larger bearing that fits the earlier Plunger brake drum. Size 1" x 2 1/4" x 5/8" Part 89-3022
These are standard sizes, but as Imperial sizes rather than Metric may be difficult to source from some bearing suppliers. Most suppliers offer a choice of brands and quality. Choose a modern shielded type, these are a worthwhile improvement over the open type bearing for this application.
Plunger Wheel Hub uses the same size wheel bearings as early S/A Wheel Hub. BSA Part 65 5883
Swarfy.