old RWS catalog gives a 18,0 mm for inside brass case and 18,15mm bore diameter for the 16 gauge cats head bullet. bullet weight is given with 48,8 gramm or 751 grains for the 16 gauge.
the bullet was also available for paper shells with a 17,10mm for inside of the paper shell and the 18,15 mm of the the upper stop ring.

here is a 12 gauge for sale in germany rigbymauser looking for reasons not to by it just now. its a very nice gun but for 2500 euro not a exactly a steal. come with a lot of reloading tools also:

got one of your 18 gauge cases just yesterday buckstix, jose from spain ask me to trade it against my own 11,5x50 PWM/Chassepot wildcat.
I was knowing the 18 ga only by old catalogs before

and when its not so hard to find 14 ga paper shells or a brass cases from time to time this is very rare. was your lathe turned brass made by RMC? reminds on lathe turned 577 Snider brass.
the 57mm long case is very unusual but with old guns everything is possible. common is to find it was chambered for the 63- 65mm or the short 40mm long brass

12 ga, 14 ga, 16 ga, the lathe turned 18 ga, 20 ga, 24 ga(70mm long brass) and 577 Snider made by NDFS in the 1990s

20 ga Vogelbüchsen exist, years ago I cut 20 ga, 65 mm long brass for another collector to 40mm for his rifle. he had a cats head bullet mould for his 20 ga. I was wondering about.
you see it dont fit into the old paper thin 20 ga.brass maybe in you 18 ga. when it is made thin like the old cases.

the bullet diameter is in between a 20 ga and a 16 ga so this was maybe for another 18 ga.