I would agree about the size of frame in the catalogue scan being the 17.5”, it looks pretty similar proportions / head tube size etc to mine  
 
 
Did you keep the old forks ? They could give a good indication of date, mine were stamped 1989 and came on a 90 spec bike. 'Spinner' made a lot of forks for a lot of bike makes / bike models etc BITD
I have a theory on the cable hanger / bridge being a different design. Might be a long shot but hear me out …
•	Smaller frame = More angled top tube.
•	More angled top tube = Shorter seat stays.
•	Shorter seat stays = Less room for cable bridge.
•	U brake for heel clearance = More cable clearance required = Less room for cable bridge.
•	Less room for cable bridge due to both above = Move it up closer to the seat tube.
•	Closer to the seat tube = Less distance / gap between seat stays.
•	Less distance / gap between seat stays = can’t make a tight enough bend to use the round tube cable bridge (due to material kinking / deforming).
Are you still with me  

  So does the theory work and could this be the reason for the fabricated / triangular cable bridge ?
Get those frame numbers run past the Marin guys  
 
 
WD  
