dunno, If the Tange decal on the fork is legit, that the fork is not original...otherwise i'd be surprised if some counterfeiter would bother to kludge up a Bob Jackson, albeit a 753 frame...why go for BJ when you could make it another "Colnago"?
Ask about more details like fork ends and what size seat post (27.4?)
but if this is a genuine BJ 753 with a decent Tange fork (and not a front end crash victim) it sure is an attractive price!