reg bonnet
You can replace just the silver bit, it unscrews from the reg itself if you take those two grub screws out - the reason i know, i had to insert a low pressure kit into my pmi preset, and those angel bottles and regs are made by PMI for WDP i do believe. So, get in touch with PMI - they shouldn't be expensive as they're isn't anything to em, just a hollow piece of metal that's been threaded.
oh and its called a 'reg bonnet' that silver part.
hope that helps, haz
p.s. don't mean to patronise, but dont take the bonnet off with air in the bottle
lol yes i assume you would never do that anyway, but hey