Dug up a few old boarding passes:

* '*30601000K09..LHG....AC.813........Y..........' - 2x on Air Canada issued BPs. Weirdly, AC813 wasn't involved as a flight at all, this was AC573 and AC8846 SFO<->YVR
* just nothing on an Eurowings flight ("SEN/SMART" printed on)
* '*30601001K09.........' a whole bunch of times when either I didn't have status yet or it was lost
* '*30601001K09..LHS....' a whole bunch when I had LH FTL
* '*30600000K05..LHS' exactly once, on a HAM->FRA BP. Note this is incredibly valuable because it seems to confirm the "05" being a length field. The thing is missing the 4 spaces at the end!

I suspect that the 01 after the 06 is the number of pieces of checked baggage, and the 000K 001K 019K 020K might be baggage weight. The 001K is predominantly on older BPs, maybe it was just a placeholder/the value wasn't wired in yet. I had 2 pieces of checked baggage exactly once in my life, I need to check if I can still find the printed BP for that (I always get printed BPs.) The mobile/web/mail BPs wouldn't normally show checked baggage since they're issued before you drop your bags...

(Need to ask my colleague about the luggage, currently travelling myself so it'll be a bit.)