I doubt it is a No 5 unless it's been made with seriously undersize connections as that should use flanged connections with 3/8" clear inlet and 1/2" outlet.

For the current No7A the threads would be 1/4x32ME which would have 5.6mm tapping size. Not sure if the old original No7 had different maybe 1/4" Whit. In any cast the builder may not have used the specified thread and Stuarts have always used a coarser one than most other makes of steam fittings use so only their fittings will fit. Stuarts list inlet pipe for the 7A as 3/16" and 1/4" on the exhaust

Edited By JasonB on 30/10/2020 07:23:15

3/16" may be the minor diameter but you will need to establish the thread pitch and profile. A bit hard to measure in such a small hole. Try screwing in a piece of round wood and measure the pitch of the marks that the thread leaves. This will establish the tpi.

Then do a Google search for screw thread specifications and find a match for the minor thread diameter. This will give you the major diameter of the thread. Then you can find a supplier for fittings.

Paul.