I would say that's a very low quality lanyard loop, in fact it looks more like it came from Numrich Arms rather than S & W - you can see the soldered joint.
I'm not certain when that 58 was made but of course before 1968 is a distinct possibility. I don't know at what point it became illegal to obliterate a serial number, I presume the only reason the loop is so far to the rear (and not in the middle) is to not obliterate the serial number - so, for the sake of discussion if the gun was made in 1965 and obliterating was not illegal until 1968, why place the loop so far to the rear?
Also, this Smith employee can somehow have a gun made up and pick & choose - but he can't have a serial # stamped elsewhere so the loop doesn't look out of place?
I suppose only the letter will tell for certain.