This is, I think, the blue frame shown in the first post? 1870 g bare frame w/o fork is what a 753 frame in a larger size should weigh. A 531 frame, by contrast, would be over 2 kg. I'm not sure about the Professional or SL version of 531.

Interesting thing is the serial number is SB1843, meaning a fairly early bike, but the seat tube is 26.8 mm. We're also told that early bikes with that diameter seat tube are not 753, because at in the early years 753 was metric and hence 28.0 mm.

And the dropouts are drilled, which I think they only did to 753 frames - although maybe there were exceptions.

So based on the weight and drilled dropout I think 753; but I can't explain the diameter. Maybe it is 531 Professional - I dunno if SBDU used that tubing?