Strange date on 5513 caseback: 1958?

[john_coburg]

Greetings all,
I would like anyone's thoughts on what appears to be an odd/wrong/suspicious date stamp on the inside of a supposedly 1968 5513, which I am considering purchasing. It appears to be stamped 'IV 58', whereas we all know (I think?!) 5513s started production in 1962. Clearly a look at the serial number would be helpful, which I am currently seeking, but has anyone any ideas on the face of this circumstantial evidence? I include a shot of the front of the bezel, which (as a non-expert) looks to be about right to me (meters first, white writing, matte dial). It might actually read '68' and the photo isn't sharp enough.
And as an aside, if I can get this watch for around $6,000 (£3,575), should I be grabbing it? The hour marks look a little worn, as do the hands, and the pearl looks to be a replacement.
Many thanks for anyone's views.
John