Anyone know what this is...?

[Agrippa]

Yeah , me too, once you get onto Belpaire fireboxes and hex frog juicers etc
you might as well be talking Klingon...

[martyn]

'Lets call it the copper thingy where the dome would have been.'

Topfeed. Its where the feed water gets into the boiler from the injectors. Most other Companies used pipes into the boiler side, usually at the front, though it did vary.

Martyn

[BudgieJane]

It's the safety valve casing. All boilers have to have safety valves, which are there to relieve the pressure before it gets too high and the bolier explodes. The fact that the top feed is incorporated into it is irrelevant.

[martyn]

It's the safety valve casing

You are quite right. I shall have to stick to Eastern Region locos  .
Martyn