Some timing covers did come with a bracket that was bolted to the cover rather than cast into the cover itself. I believe they were towards the end of the 70s. Most likely yours had the bracket and it disappeared.

Which is a good clue that some or all of the following may be true:
1) The engine is not original (or at least the timing cover is not).
2) The engine may have been rebuilt
3) The camshaft may have been swapped


RamAirRestoration sells a fancy billet replacement if you're not satisfied with scribing the cover. I think there is a little adjustment to their piece, so you will still need to verify TDC and align it properly. That's always a good idea to check anyway.