4 vs 5 digit references

Something's always confused me about Longines.

Sometimes some models don't have a 4 digit reference on the caseback, but instead have a 5-digit number starting with '2', as in this example here. In that example, Longines helpfully came back to say it's a ref 4638, bearing order number 22155.

In another example on ebay however that was recently sold (original link here, but I think it doesn't work any more) , seller says it's a ref 4638 too, but the case back says 21411, and the Extract says it's a ref 4638 (with no reference to an order number of 21411)



So I'm now a bit confused:
1. for those with 5-digit casebacks starting with '2', were they all linked to a clear 4-digit reference number? Or are there 'orphan' 5-digit codes with no corresponding 4-digit references?
2. in the case of the ebay watch with pics above, seller said he was positive ref 4638 matched his caseback of 21411. Not sure how did did that when the archive didn't say so - could the caseback thus be from a different watch?

Any info would help - the ebay watch is sold, but this still puzzles me, and if anyone could enlighten me, advance thanks!