My memory isnt what it used to be 20 years ago, so I measured some seals to verify my statement.
Bear in mind that I find measuring seals accurately difficult, but these were all measured by me with the same cheap digital calipers, so the comparison should be valid:
BSA black seal with some, but not very much wear: 29,15+mm
Maccari BSA seal, has been tried in a gun but not shot much since the tube was too big: 29,2+mm
Knibbs BSA seal, unused: 29,7mm