They were the original manifolds on the 70's Glastron jets.   They did have a flapper.   I went
to the long bolt manifolds myself.  I did however have no choice since I went with a stud girdle and needed tall valve covers.  I don't know who made them.
Look at the hoses on the old engine, the entrance is the top right of the timing cover.  Exit to the manifolds
was from the thermostat housing.  The small bypass hose was to keep water going to the manifolds when the t-stat  would close.
You are right on with your hoses.  That set up showed more temp swing than the logs at least on my jet.