Well I'll be dipped. You're right.
All these parts are Hardware (fasteners),
they all start with a '9' and end in '00'. - 9XXXX-XXXXX-00
9XXXX-XXXXX-00
The second and third digits tells us what type of fastener:
0 - 'various'
1 - bolts, cotter pins
2 - screws, nuts
3 - oil seals, O-rings, bearings
4 - spark plugs
5 - bolts, nuts
6 - ISO bolts
7 - ISO screws and nuts
9XXXX-XXXXX-00
The fourth and fifth digits tell us what material/surface finish (e.g. chromed).
9XXXX-XXXXX-00
Digits six and seven give the diameter (inner or outer) e.g. 08 would be an M8 size.
9XXXX-XXXXX-00
Digits eight, nine and ten are the length (except for '901' bolts for some reason)
There are also standard fastener types...e.g.
90105-XXXXX are Washer based bolts
90116-XXXXX are Studs
90401-XXXXX are Union bolts
99999-XXXXX means that a part has been superseded by a kit.