Looking at some information on BSA serial numbers recently & it seems that 177 mk1 Mercury's have a W prefix (1973-1979) & mk2 Mercury's in 177 have WB prefixes going from 1974-1978. My question is how reliable is this? I know that barrels can be swapped around but I've come across 2 of them (mk 2's) with NG prefixes that seem to be allocated to Meteors. Guess both could have been swapped out but seemed a bit of a coincidence to come across two of them which made me wonder if there were more, possibly a short production run with barrels to hand that had been destined for Meteors but got used to fulfill a Mercury run. Any thought?