If it was about 3-350.00 then it may be a possible buy. £1000 and even £650 is just too much based on what has sold this year on eBay. It has a really poor and very thick repaint, perhaps why the SB isn't visible. The image from under the BB looks incredibly thick. Even the person writing the advert said the rear dropouts look like Campag - they probably can't see campag on the dropouts because of the bad paint.
Unless you have the frame in your hands and can measure the tube diameters you may never know about it being 753. Ask them the weight of the frame and forks, there is a definite weight difference between 753 and 531. Ask them to stick a seat pin in and ask what size pin it takes. It has all the signs of being an SB frame. The 3 top tube cable guides aren't the norm but could have been requested from new or changed at any time in its 30 year life. The head badge probably got changed as part of the poor refurbish.
It's my size, 57cm, and if I didn't already have one, I'd be tempted but not at that price.
There are plenty of people riding old 753 frames like this - but with a 30 year old frame and no history, you always take a chance.
Good advice there.I guess i would like you say would have to look at it close up because of the paint job and not identifying the markings.I think at some point the purchase will end up being a bad one.