Rolex Submariner 1680 advice

[Treborian]

Advice needed.

I’m on a hunt for a 1680 from my birthyear 1976 and found this one. It looks good but I’m having second thoughts about the lume and a few other things. The lume seems inconsistent and uneven. It’s my first vintage piece (I already have a Explorer and Explorer II) so I’m no expert in vintage by any means.
Except from the lume, the case and dial looks OK but the insert looks strange with a square ”4”. It doesn’t look like a MK1-3 or even a service insert from that era. The watch was sold in the US in the end of the 70’s.

Please chime in with comments and advice if possible.

Thanks