There are two ways of getting a brass master. That is to make it from scratch by soldering various panels together etc. There are a lot of downsides to this method though. Or, you make a resin block of the outside and then get a casting in brass from that. The red one you see here is, I’m pretty sure, a resin block that has been primed to give an even finish before a mould is made to cast a brass master. - Darren Mould, 2022.