I was able to purchase a Richard Penney Boiler Cap for my '67 Cremina from another member - Orphan Espresso appears not to carry this at present. Anybody know the thread size for the top (BPP or NPT?) where the Manometer would screw in?

I cannot imagine it being anything other than NPT and I would suspect that it is 1/8"-27. A simple trip to the local hardware store can confirm that.

An excellent source for gauges are these guys: http://www.valworx.com/category/gaugestore

Yes, it is definitely NPT threads. The boiler refill hole on the Cremina has BSPP threads so that's why there is a market for a boiler cap adapter from BSPP to NPT. In other words, if you bought a manometer with BSPP threads instead, you wouldn't need an adapter fitting to NPT (like we talked about a while ago in July Richard Penney Boiler Cap for Manometer ).

Don't remember if it's 1/4"-18 or 1/8" -27 but you can just measure the hole and find out.
1/8" should be 0,405" or 10,2 mm if I understand this table in wikipedia correctly:
http://en.wikipedia.org/wiki/National_pipe_thread
(I don't know enough about this so not sure if standard NPT threads are the same for OD on a pipe compared to male/female threads on fittings)

The thread profile for British pipe threads is 55 degrees, and the tops and bottoms of the threads are rounded. The US ones are 60 degrees. If one examine them with even slight magnification it is possible to tell the difference between those and the US ones if you know what you are looking for.

Thanks all. Down to the hardware store to measure, then to purchase manometer.