Apologies for the late answer, ac12, I have been very busy with work lately.

I was not able to find a full copy of the manual. I did find an article in Camera magazine, December 1967, that describes the operation and calibration of this meter. My local library happened to have a copy of this article. The calibration procedure relies on a calibration negative, but, as you suggested, the same result can be accomplished with ND gel. I obtained ND gel in different grades, then work got very busy, as I mentioned, and I have not been able to test the calibration procedure yet.

I can send you a pdf of the relevant pages.