That looks like a Mamod set and normally you would remove the safety valve to get the water in and remove the screw in the end to establish the water level. i.e. when water comes out at the end then the boiler is full.
I'd be worried about the quality of the apparent repairs with soft solder along the top and the end. If it were mine I would carefully remove the existing solder followed by cleaning and resoldering with a good quality soft solder.
The safety valve would need to be tested to make sure it prevents over-pressure and the boiler itself would need a test just to make sure that everything is safe.
It will look nice when restored though. There must be some other Mamod enthusiasts on here who could advise,
Brian
Beaten by John & Martin!
Edited By Brian H on 26/04/2020 16:58:07
Edited By Brian H on 26/04/2020 16:58:53