I received two new seals from Moss on Saturday and then proceeded to re-work the installation of my adapter. I suspected that the groove in the oil pump head was filled in by a very old seal. However, on closer inspection with a brighter light and cleaner mirror I discovered that the groove was there and not filled in. After cleaning up I tried fitting one of the new seals in the groove. It appeared to be too small. I tried the second seal, same thing. I compared all seals and they are all the same. When inserted into the groove it was not possible to get the seal to sit in the base of the groove all the way around. I tried stretching the seal without success. In the end, used a little axle grease to keep the seal in position and installed the adapter with a bit of green Loktite. I've got no idea whether or not the seal was successfully pressed into the groove. However, if allowed to determine this by the lack of any oil leaks, I'm guessing that it did.
So, I guess this will close this little adventure until the next oil change. Thanks for all the comments and to Moss for their help.
Dave.