You're on the right track, but not quite right.
The first time I saw this system explained was in TRF's TR250 manual.
HU is a "set screw" - fully threaded.
HB is a "bolt" - partially threaded.
HN is a nut.
JF is a jam nut (thin)
LN is a slotted nut (castle nut)
TN is a thin nyloc nut
YN is a thick nyloc nut
WL is a split lock washer
WP is a plain (flat) washer
By default, it's grade 5 hardware. Bolts that have part numbers outside
this system are special in some way - stronger, or have a shoulder, or
an unusually threaded section. For example, the bolts that connect drive
shaft flanges are not in this system (and I prefer to use AN hardware
for them, but that's a different discussion).
For bolts, the first digit indicates the *wrench* size, and the
second/third digit indicated the length in 1/16" increments. So HU704
takes a 7/16 wrench (so it's a 1/4" bolt) and is 1/2" long.
For nuts, the last two digit indicates the wrench size.
For washers, the last two digits indicates the wrench size of the
associated through bolt/nut.
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