It sounds like we were both doing the same mental gymnastics! It also occurred to me its maybe possible to determine how many Warriors were produced by using this sequential engine number. Hopefully someone with a 2009 will post and the sequential number will follow suit lol. The Road Star jugs differ, but I don't know if the MT-01 engine is 100% identical and if that might muddy the water do you think?
For what it's worth , I would guess that AZwarrior's spare is an '04 model.
This deduction is based on the info above that shows that the later the year , the higher the portion of the engine # that I highlight in red below
Engine # P611E-0012497
I suspect that this is the actual sequential engine # as relates to when and how many have been built. I don't know if they started at '1'.
The # on AZ's spare is closest to 04 Indigo's # as seen above.