This is intriguing.
The Reynolds catalogue page attached shows this particular fork decal with the 731os frame set. However, the part number, EB708, is also used in 708 and 653, albeit with a decal to show this. Looking through the 1991 catalogue the exact same fork blades are used in 531 Designer Select (for what they call Paris Roubaix, strong rider, rough roads) and 531 Mix and Match, albeit with a fork decal in these colours but with '531' displayed. The second attached scan shows what Reynolds called their variable technology, essentially using 531 at the front end. Here's a photo of my Bob Jackson - we're about to depart to the south of France for 3 weeks (my partner also rides a BJ).
Exactly what these forks came off may not be solvable but decent forks, although my money's on 731.
Good luck.