Well, I may as well expose myself in public about the whole process here. So here's what I've found so far.
First, again, thanks to everyone who has sent scans and pictures. They've been incredibly helpful. I could not have gotten this far without them.
What I have found is that there are *TWO* configurations for the lower standard for the Laborator 1000 enlargers. I have two manuals, one from johnnywalker and one from KenS. One matches what I have exactly, the one from KenS, while the other from johnnywalker looks like a "newer" version. I'll call one "older" and one "newer" to keep them straight, but I might have them mixed up chronologically.
The "newer" version has a red filter that protrudes from the standard and the Lapla board attaches directly to the standard. This standard has a round hole in it and the little prongs to mount the Lapla.
The "older" version has the oblong hole that I show in my pics above, and obviously the same as Peter mentions as well. What goes in this hole is one of two "connecting rings" that hold filters. The red filter version is called simply "Connection ring with red filter mount." (Wow! Who came up with that fancy name?) This is clearly the one for B&W work. The second option is a thing called a Tauport which has three filters that mount *INSIDE* the bellows unit. I haven't quite figured out if this replaces the "connection ring" or just replaces the red filter on the "connection ring". The manual states that the three filters man be used for 1)Color enlargements according to the additive process with three normal filters, 2) Production of color separations, and 3) Enlargements upon gradation paper.
So, the 104mm diameter mounting prongs I have are not where the lens board proper mounts, it's where the connection ring mounts. Then the 78mm lens board mounts to another set of prongs on the connection ring.
My guess is that the double mounting scheme, with the thumbscrews right next to each other, proved to be a mechanical mess for people. I'd bet that's why things changed to the "newer" mounting scheme.
Of course, now I'm looking at finding a connection ring, or fabricating a part to cover the hole.
Michael