If you are not aware, the BFD has two sizes of focus knobs! At some point in its history, it changed very slightly.
Thanks Alec No I was not aware of the change .The size may have changed when Jim removed the 2 small focus pointers that were replaced by a Red marker on the handset.?
As I own 8 BFD's sn0023 and later I have found that the difference in size has no effect on the disc's as they have quite a "spring" to them and lock firmly around all of my hand controls. Even without the coverplate and screw to lockdown the disc 100% in place( to stop any sideways movement) by having the keyway screw the disc very nicely just sits in exactly the same place every time.
The BFD SnapOn Focus Disc's will be available in early October so if you would like one just email me ok .
Fly safe stay sharp