TH44:
First let me endorse all comments already posted here: what a marvellous thing! You must be very pleased to be looking after it for the time being.

That said, I would be loath to modify the chambers when all that is needed is a form die to be used only once per case. After initial forming the standard sizing die would neck-size for the life of the case, given the relatively low pressure involved.

A competent machinist should be able to make a form die using a boring bar or by making a D-bit, given that overdoing it slightly won't matter because the case will fire-form on first firing and can be neck-sized thereafter.

Not sure whether CH4D will make a custom die from a chamber cast, but that might be an option. They are happy to export at least!

Back in the day (10 years ago?) Jim Goodwin of North Devon Firearm Services was the solution for custom dies when Peter Davern retired here in Oz, but I know Jim has long-retired also and the NDFS name is being used in the US now, by a company that refuses to export outside the States. They might still be an option, but you would need a sympathetic forumite to do the transaction within the US and then ship the dies to you, as the new NDFS emphatically refuse to export outside the States!