Kevin, I think that Brown & Gammons in the UK also uses original MG part numbers, which I presume would be the same as Sprite numbers. Of course, there are a lot of parts on your Bugeye that I believe they do not carry. I have heard their paper catalog is worth ordering like any other book on Jack's list, but it is not cheap.
You have to think about how these parts are numbered in terms of how the parts are binned in Moss's warehouse. If you peruse the Moss's website long enough you might begin to discern some kind of logic. It seems like the six number code for nuts and bolts starts with 3XX-XXX. Brake parts begin with 180-XXX, etc. If you looked at it long enough you would see a pattern.
So, if you are a stock picker with an order printout, your job is to pack the order, you go to the aisles with the 180s, pick out your brake parts, move along to the 300s, for bolts, etc. and fill out the rest of the order. You want someone to be able to do this really fast.
Of course, Moss is also using this same six-letter code to organize parts for Big Healeys, Triumphs, MGBs and MGCs, etc. so they don't have to have a separate warehouse for every make of car, or 10 different bins all over 10 different warehouses for the same size screw.
If you download that parts list from spridgetguru.com you will see that the parts are numbered almost randomly, probably because they were borrowing from so many makes of existing BMC cars to build the Spridgets. I should imagine if I was going to start from scratch I would come up with a system similar to Moss's.
Of course, part number taxonomy, binning, and warehouse layout is an entire science--just ask Amazon. They are probably the best in the world at this.
Just my 2-cents' worth
Charlie