Bob,
The knife was made in the mid 1960's, in my opinion, no farther back than about 1964. The sheaths have been stamped in numerous locations and this one proves they can be different places. There is a possibility that, realizing nothing had been stamped to indicate blade length (or model #) Johnson's put these in, after-the fact?? Who knows? 1950's Model #1 sheaths would have been Heiser-made, brown button mostly.
Hope this helps, Capt. Chris