Capt. Chris--

Your logic is that it is a Model 1-6 (with the Model 1 "top" not being added to a Model 5 per Shop practice or policy).

Which makes sense because why add the divot that characterizes the Model 1 to a Model 5 anyway?

And it is left for us to assume that somebody wanted Thumb Notches added into the Model 1 divot. Perhaps that somebody "thought" that the divot plus the notches would provide improved purchase for the thumb or finger that is placed forward of the hilt.

Since it apparently has never been a "user," perhaps the original buyer simply wanted something that nobody else might have ever ordered, but that person probably did not foresee that he would create a mystery to intrigue and befuddle us of the future!

Larry