If you have an Al-KO axle with standard hubs the dust cover for the hub nut will have ALKO stamped on it, the same is true of the grease cap of older units.
The brake shoes are determined by the diameter of the drum and the width of the brake shoe.
So, if the drum diameter is 200mm and the shoe width is 50mm, the set needed is 2050.
The other sizes Al-Ko do are 1637, 2051 and 2361.
BPW and Knott use different identification markers, with some shoe sets common to both BPW and Knott.
Look here for the best place to get them from:
https://www.westerntowing.co.uk/acatalog/All-Parts-for-Brakes.html
When working on the brakes make a note of how the springs are fitted before removing the shoes.
Release the tension pins and keep safe with their spring clips (although new ones should be included in the shoe set).
Remove the shoes then give the whole backplate and everything still on it a soaking with brake cleaner to remove all the dust and rubbish, also thoroughly clean the brake drums and check for scoring.
Reset the adjuster at the bottom of the backplate before refitting the shoes, ensuring the unit is free turning and lightly greased with copper grease.
Lightly grease the brake expander at the top of the backplate.
When refitting the shoes it is easier (but still a bit fiddly) to fit the springs first to the shoes then get the shoes into position, making sure you have them in the right place so that the expander operates correctly and the adjuster end sits in its slot of the adjuster..
Refit the drums and adjust the shoes via the star wheel which is got to through a small hole in the backplate.
There is an arrow showing which way to turn the star wheel to tighten the shoe to the drum. (so its a lay on your back on the floor with a small flat bladed screwdriver)
Tighten the shoe so that the drum is locked, then back off the adjuster by 13 "clicks"