No reason not to take this at face value I think. The Strada was mentioned earlier in this thread and looks a distinct possibility. The frame doesn't have the typical features of a TT frame of the time such as vertical dropouts, shot in/fast back seat stays and I assume no drilling out of the BB, as per catalog. But there no reason that it couldn't have been kitted out for TT use. I guess the frame measurements would confirm this. If its had little use then the paintwork is probably original too. If TQ was working at or for the shop at the time that would fit too he would have built plenty of earlier shop Strada's and other Italia models.
As for the frame number/date, I have now seen two other RT with 4 digits and then some with 5 digits and often with a letter. I think it fair to assume he used Holdsworth shop numbers up to 1975 and then his own, so this would have to be post 75 I would have thought. If an early one could have been built during 75 for later sale. With so few numbers and no really good date fixes it is difficult to know how the number ran. The highest number I have got so far is 57150, way too high for a sequential shop build rate starting a zero. So either there are some changes in the numbering system or some built in coding, or he may have had access to some factory supply and badged them as RT?
I have started a RT database for my own purposes which you can view here if you wish.
https://drive.google.com/drive/u/0/fold ... TNKZkN4TmM
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On the road: 1991 (Falcon) Holdsworth Professional | 1989 Raleigh Road Ace | Mid 80's Tom Board Lo-Pro | 1980's Roy Thame? Sprint | 1980 Holdsworth Professional SL | 1980 Holdsworth Special | 1969 Holdsworth Italia Sprint | 1958 Holdsworth Monsoon | 1958 Holdsworth Zephyr - shop special| 1953 Claud Butler Olympic Sprint
Others: 1996 CB Criterium | 1982 Holdsworth International Sprint | 1976 Holdsworth Mistral | 1974 Roy Thame Cronometro | 1966 Claud Butler Coureur | 1965 Holdsworth Italia Strava Speciallissimo | 1958 Holdsworth Zephyr - Nervex Pro Lugs | 1958 Zephyr | 1954 Claud Butler Jubilee | 1950 Holdsworth La Quelda | 1948 Claud Butler International Path | 1934 Holdsworth Stelvio