It looks like they were trying to just cover up the Selmer logo.
Maybe an embarrassed British teen who didn't want to be seen with the Marshall stack's pathetic cousin, or maybe someone who had a brand endorsement and needed to not be seen with another amp. It could also be like a 2 foot long 6 inch wide sticker.
That the first thing would be a shame, because Selmer amps are awesome. Did you know that the Animals used a Selmer amp on the House of the Rising Sun? (Or at least that's what I read on the internet, once!)