The 1/8" pipe plug is factory and I believe to be only on motorhome heads. I just sold a set like that a few weeks ago. I removed them from a 440 that came out of a motorhome.

CHIP
'70 hemicuda, 575" Hemi, 727, Dana 60
'69 road runner, 440-6, 4 speed, Dana 60
'71 Demon 340, no drivetrain, on blocks behind the barn
'73 Chrysler New Yorker, 440, 727, 8.75
'90 Chevy 454SS Silverado, 476" BBC, TH400, 14 bolt
'06 GMC 2500HD LBZ Duramax