S4 vs S5 N/A Air pump differences
 

Are there any differences in the air pumps themselves, brackets and bolts?

Looking at the parts diagram shows they have different, but very similar, numbers for things.

My main questions pertains to the top bolt, as it's apparently discontinued for the S4, but not the S5. Also, the S4 has a split lock washer and regular washer but none on the S5.

Thanks!

the S5 bolt should work fine, all of the 1981-1992 air pumps are the same, but the hose fittings vary, depending on the engine/car

Mazda part numbers work like this:
N304-13-710
first 4 digits tell you what it originally fit, N304 is an 84-85 GSL-SE, N370 is the S5 US turbo, N374 is the Japanese turbo, etc
the next 2 tell you what system it is in, 13 is intake and exhaust, 15 is the cooling system, 16 clutch, etc.
the next 3 tell you what it is, 13-710 is an air pump, 15-200 is a radiator, etc
if there is a letter, it tells you if the part has been changed. it goes A-Z.

so for an air pump, the 13-710 part of the part number will be the same, it just tells you its an air pump. the N304/N350/N370 tells you what it fits, and you can go from there