The rear dropout is a Campagnolo portacatena which dates the frame to 1977-78 - they weren't available long.
While 753 was first available in 1976, builders (as has been said before) had to pass certification by Reynolds to prove they could braze/solder it without weakening it.
Being an extremely thin-wall tube set, thick paint could be hiding rust damage. The seat cluster gives me doubts that this frame is a Raleigh, I much more doubt that it is 753.
Anyone who'd put a parts-bin build including a seat post of that "quality" on a 753 frame would not have my confidence as a mechanic.