There were 2 designs of saddle stop (used for turning up to a shoulder or boring a blind hole) for the 5" lathe, both of which should also fit on the 280 series.
ML104 was a simple, non-adjustable stop whilst ML105 had a micrometer adjustment.
See drawings below.
There was also another device called a 'saddle stop' on the drawings (ML116 and SK597), but this was a safety device as far as I can tell - it had a switch to cut out the motor if the saddle was wound into the headstock.
Any photos of any of these items would be good to add to the archive!
Andy