Not C&R eligible? Explain.
From the BATF C&R website
"...ATF has recognized only complete, assembled firearms as curios or relics. ATFs classification of surplus military firearms as curios or relics has extended only to those firearms in their original military configuration. Frames or receivers of curios or relics are not generally recognized as curios or relics..."
However, you can ask for the BATF to place a firearm or group of firearms into the C&R catagory based on rarity, unique or historical provenance, ect.
Perhaps that was done with these few remaining frames.
The addition of the ser# and being sold as a 'kit' may have something to do with it as it may then be closer to being considered a complete firearm by BATf . Just a guess of course.
A shotgun frame alone would not be C&R,,
and one mfg before 1968 would not need a ser# on it,,only that it could not be possessed/sold with an obliterated or altered original manufacturers applied ser#.