Your set with PH, PE, PF, PI would stack up with mine then (apart from one which was a from the previous year), as I’d expect Shimano built up a stock of cogs over the preceding months from which to assemble cassettes as needed. Unlikely they made the effort to ensure or cogs were exactly the same age.
As for the NOS set with WJ & XF cogs…that would suggest they are from ‘98 & ‘99 if the build date sequence continues, which seems way off for M900. Could those cogs be from M950/1/2/3 cassettes? Are they even the same colour/style?
Exactly my thoughts mate. I imagine there are giant bins of them and Japanese children under 10 assembling cassettes for $2 a week and shipping them to the UK.
I think my lock ring is correct but more than likely the last two rings are donated from a 737 cassette which would have the right sizes. Not the best find on my NOS boxed M900 cassette I paid for but an occupational hazard I guess. At least they are new.
Finding this interesting regarding cassettes, as there is one sitting on the side I thought I'd have a look. 12-32 type P, fully riveted so later. Has codes between YI-ZE which puts it much later, September 2000 to May 2001.
This was boxed NOS when I bought it, very surprising that it's so late!
According to Mombat the M900 cassette was an option through the M950 years as well, makes sense, not everyone would have seen the point in/wanted to pay for the ti sprockets on the M950. Then I expect they carried them on for a bit afterwards for spares.
Different date codes in the same cassette, they were probably shipped to Madison in separate bits and they just grabbed the right sprockets out of a box to make up a cassette.