Ben... Deffo not the correct wheel. Speedo drive box on the rear spindle was never a feature of the pre unit A and B ranges, they are all driven from the gearbox. Bolt on sprocket is also later, along with a ribbed brake drum. Wheel rim size is also different on many later models, 18" rim used on A50/65, and Unit B and C Ranges. That tyre looks to be a modern profile 18 "size, if my specsavers eyes are working...can make out what looks like the botton of a figure 8.
The hub crinkle pressings are the same as the earlier bikes, as is the central tube internal diameter. If it was the last wheel in the world there is scope for mix'n match, using the correct hub internals and losing the speedo drive. There'll be a lower rear ride height and the rim offset may end up where it shouldn't be, and at the end of the day I'd suggest a continuing search for the correct parts, armed with your updated knowledge. That brake drum won't match up with a plunger or rigid backplate, the ball races and spindle diameters are different sizes.
Swarfy.