I have a Bob Jackson with the serial #1844 and I have been told by the people at Bob Jackson that their master sheet lists that bike as built in 1956 "for production". It also has two other numbers, #143 on the back of the steerer column and underneath the #1844 on the BB and #60 which is actually on the end of the #1844 on the BB but not on the steerer column. These extra numbers are probably "renovation numbers" according to Bob Jackson (and my own research). It was normal practice for a "re-spray" shop to stamp their own numbers on a frame before it was stripped for repairs/repaint. My own frame was built with Simplex dropouts and the rear derailleur hanger has been modified for a Campagnolo Derailleur but has the same paint as the rest of the bike over it. Probably modified in the early sixties and had a fresh coat of paint and transfers applied.

vjp