As Jorrel mentions it's entirely possible that it's a broken spring. It was common on that vintage. It won't be seized - I've never seen that on these engines.
The other possibility, although rarer, is a dislodged valve seat. But I'd go with a broken spring. It can be replaced with the head installed, either with compressed air into the cylinder or the rope trick.
I have never used the rope trick myself, as I will never introduce a foreign material into a customer's engine, and never will. The best method is compressed air.