~~~ your guess is a good one; I have also seen several chips and have pointed out here that a simple part of a graphic can be matched with another denomination of a series that the chip can be part of or, a distinct border or ring matching all other denominations of a series and, last year I posted an image of a "2f ⁵⁰" MOP chip on both sides with nothing other than the size and font of the "f" might match something and, just a few moths ago I posted a complete series of MOP chips and plaques from the 1000 down to the 2 ⁵⁰ where all denominations in the series had a monogram on the obverse while the 2 ⁵⁰ was engraved on both sides and pointed out that the 2 in the 25 and the 5 were exactly the same font as the 2 ⁵⁰ chip... and, if I come to an agreement with the seller, I can easilly present the 2 ⁵⁰ chips as being part of a defined series...

Excellent question, by the way.