The moulds hardened up nicely and opened without ripping,
A little trimming of rubber from the hole I cast through and they get filled with resin plaster.
This is the nerve racking part, particularly because I'm sharing my potential failure online. I cast one of my 19th century casualty markers at the same time, always best to do as much as casting I can at any point to avoid wasting the plaster I have mixed up.
I noticed too late that the air release paths I'd made in both moulds are blocked, the casts might still work but I need to trim the mould to open them up before the next time I use them. Ooops.