The BYMS ( British Yard Minesweeper ) was an RN order for USN 'YMS' type motor minesweepers. Unlike their RN MMS couterparts, they were not based on a fishing boat design, but were designed as warships, with all the refinements in design of one.
BYMS.81 - 250 were absorbed into the USN, but 70 were re-allocated to the RN, an renumbered from 137 up, but with a lot of exceptions. They had the pennants of minesweepers - J - with 800 added to the unit number.
This was a very succesful design, heavier armed ( a 12pdr AA gun was shipped ) than the MMS, and thus able to be deployed in more dangerous waters. It was not as sturdy as the MMS however - the latter's fishing boat basis proved extremelly tough and dureable.
The original 80 ships were different from the subsequent vessels in that the two exhausts led to seperate funnels, the latter units' were combined into one casing.
Postwar, 2000 was added to the unit numbers ( i.e, BYMS.19 became BYMS.2019 )