I think its braking force to weight ratio.
  if they dont have a weight chart, or if the bike is modified, they weigh the bike.... 
 am suprised they didnt have a modern bike chart for your bike
 
  my Lawson was weighed back in Aug for its MOT, but is 80's bike - and heavily modded 
 HTIOH 
 AK 
 [Edited on 22-10-2005 by CK and AK]