A rare old Lum is currently on Ebay. The seller indicates that this was the 13th knife made by Bob. A couple of indicators to support that claim.
1) Maker's mark is the Chinese character for Lum hand etched.
2) The sheath is identically marked.
3) The sheath work is consistent with Bob's style.
4) Handle hardware is consistent with a knife from that period.
5) Mirror polish would be the standard finish for the period.
6) Taper tang with liner.
Good luck to the seller and the new owner.