What memory size option was specified in the SHN Utility when the link records were displayed, Small or Large. It sounds like the Small option was used (the default) which would be consistent with not finding link records at 3FF8 (Large option under SHN Utility).
I have not seen a case where the memory size algorithm failed to show the correct PLM memory size. The Set button method of creating a link record is an absolute test. The internal PLM is writing the link record in this case and will write it at the location associated with the top of the memory installed. The SHN Utility can be used to verify the link record location. For a large memory device the SHN Utility will show the link record with the Large option and no link records with the Small option (assuming a Factory reset preceded the Set button link to insure all link database memory has been erased).
For a device that has the small memory the SHN Utility will display the link record using either the Small option (Set MSB 0F) or the Large option (Set MSB 3F) as the device memory will wrap the Set MSB 3F value back to 0F.
It would be good to know the revision level of the EZIO2X4 and EZIO4O. Since the tester is using the SHN Utility, simply go to the EZIOxx tab, enter the appropriate EZIO Model and enter the Insteon address of the device. The firmware level will be displayed below the Insteon ID field.