http://www.skgrimes.com/lens-mountin...ed-to-shutters , towards the bottom of the second screen down.
I just compared mine with that image and the visible bits in the image look like it. Mine has a groove in front of a ~ 20 mm tube with slot for Waterhouse stops in front of the part of the barrel with the diaphragm. I think that groove is joint between front cell and tube. It matches what looks like a front cell in grimes' image. At the rear, mine has a threaded section with a slight lip that abuts the rear of the barrel just behind the diaphragm control ring. This matches what's visible as a rear cell in grimes' image.
Charley, thanks for the kind offer. I also have too many 300s. Apotal, Apo-Nikkor, Apo-Saphir, TTH Telephoto, ... I don't know about you, but although I don't have as much coming in as I did before I retired I can still afford the odd interesting and inexpensive lens. It seems to me that the supply of interesting inexpensive lenses has more or less dried up. And I say this even though I recently couldn't make myself pull the trigger on a couple of very inexpensive Zeiss Anastigmats and am ignoring some ancient and pretty Perigraphes. I have good enough lenses of those focal lengths, can't see the point of adding more.
The money is better spent on cookbooks, although, again, I just can't make myself pull the trigger on a $1,100 set that I'd somewhat like to have. I have all of the second edition, would like to compare with the first ($1,100 for 27 of 33 volumes) but not that badly. I've told my favorite library about the set on offer, hope they have enough acquisition budget and will to buy it.