Andy, your bike will be pre 1981. I was in charge of frame design and production at Falcon in Brigg during 1981 and I remember we decided to braze on top tube cable guides on all frames. The dealers loved the Shimano 600.
Not top notch bikes but actually all hand built and value for money. Some of the builders, both male and female, would hand braze more frames in a week than a custom builder would in several years, consequently they had very high skill levels and I had no qualms about them building 753 frames for the Falcon pro team. When I was there there was a team of around 12 producing 400 frames a week.