dwscrimshaw
Retro Guru
Not sure I can help with RT numbers after 1975. The ones collected are mainly 1975 shop built with a few in 74 and a couple of possible others earlier than that. Holdsworth then changed to the six digit number mainly in the same location as yours and always with a leading zero.
They come up on Ebay regularly often with numbers, but there is only one currently listed as 70's. That seems to have an earlier number than yours if he used sequential numbering. There is an other listed as 80's with the same lug work as yours. I've not seen them with the number in this location before so maybe he moved it latter? The ones I have seen are under the BB, like the attached.
Sorry not to be much help, but I am only collecting the ones that fit with Holdsworth.
They come up on Ebay regularly often with numbers, but there is only one currently listed as 70's. That seems to have an earlier number than yours if he used sequential numbering. There is an other listed as 80's with the same lug work as yours. I've not seen them with the number in this location before so maybe he moved it latter? The ones I have seen are under the BB, like the attached.
Sorry not to be much help, but I am only collecting the ones that fit with Holdsworth.