I think you nailed it, I just hadn't seen them on a rod before.I think the objects in question were a pair of Starrett 271 v-blocks with the alignment rod. It doesn't belong with the head. I don't think you'll miss them although they do come in handy every now and then.
Starrett 271 Precision V-Blocks - Higher Precision
271A STARRETT 271 Precision V-Block: 1-1/4"www.higherprecision.com