Crusty, I love your rifle! The engraving appears rather stylish. I got my Cerrosafe here:
http://www.buffaloarms.com/browse.cfm/4,1143.html

Its an excellent medium, but you can also use stuff like Sulphur which can be had at a local garden shop, today, for less money. Sorry, I don't know what its shrinksge factor is but not hard to find out.

Use a micrometer to measure, I wasted time with a vernier but it just messed me around. I got a good one off eBay actually, but so many people can let you use theirs for 20 minutes.

My bore slugs were made by using 11/11.5/12 mm drills in hardwood, melting a 220gn cast bullet and pouring the lot into the hardwood hole. take care not to try and beat too big a slug through, little is easy. Best if you can get one is a round ball of pure lead such as you would use in a .50 patchball M/L rifle.

Until you have the cast measurements, it can only be speculation what cartridge it uses; but with a vernier you can measure the rim and body diameter at the mouth of the chamber, the rim depth, and the depth to the case mouth step in the chamber. That woul dgive a couple of clues.

If you don't have COTW, try here for an incomlete Google books preview:
http://books.google.com.au/books?id=UoWa...44HOY2wK5IOQGhw

Or here for a good source to buy it: http://huntingbooks.com.au/

I have to say, I was really pleased with Bruce Bertram having the cases I wanted, in stock and ready to ship. Although 20 of them cost $95, the alternatives were all tedious and unsatisfactory by comparison.