Excellent find Geoff, it's a beauty even in it's present condition and well worth restoring. Congrats! My guess would be that the 2 indentations have tapered set screws in them that have had the slots twisted out; and somewhere along the line someone thought the indentations were to insert a case wrench and twist it off. That would explain the deep scratches; as it appears the worst damage is on the "counterclockwise" sides of the indentations. There could be a second threaded back under the outer caseback that it is screwed to; to make it watertite. I have a slightly newer Lamont (another Swiss) that has set screws next to all 4 lugs to cinch down the waterproof back...and all 4 set screws are toast.
I researched waterproof watch case evolution/history a few years ago, and I think if you are patient and google the right combination of words, you can probably find the patent info on the case with drawings that will indicate how it's put together.
PS - Lke your band choice, it's pretty close to orig in appearance.