I just found my old one which I bought from Jim Jasek back around 1973 or so. It was in a closet with some old Army stuff and is still in great shape even if a little mud stained.
I think this was discussed here a couple of times for its use in calculating multiple flash exposures in large rooms, etc. While I don't have the manual anymore I remember a complete writeup with instructions and examples in the Speleo Digest of 1970-1975 era.
The TFC is essentially a circular slide rule device 4 in in diameter. It has a table with Guide Numbers and BCPS for flash bulbs, permits you to calculate the GN based on film speed, shutter speed, etc., etc.
Anyone in Huntsville Grotto interested (keep it in the family)? If not, it goes to anyone else interested.