Thanks for the images of your construction process. Very nice work.
I am far from an expert, but I had always thought that the Spanish and French brass pulverin parts were bulk cast sets and that they were run in batches. This would allow for some small approximation of mass production and part interchangeability. And the castings would be purposely designed to fit an average sized horn. Much like the brass pulverine castings you would get from TRS today, sets are all the same size one to another and after fitting and finishing the castings, an average horn is all that's needed to complete one. Finishing the castings is the real work, fitting a piece horn between the end caps is pretty easy after that.
The idea then is the horn is cut to fit the castings not the other way around, at least for the basic military contracted type stuff I would think. High end pieces, or even country made home spun versions would likely require custom made fittings. I.E., if you only have one horn, you will make the brass ends to fit it, but if you have a hundred matching top and bottom cast sets you will fit the horns to them. I had always thought this was an evolution in production methods to speed manufacturing in order to better outfit bigger armies with standardized stuff. This would be a step up from the older wooden trapezoidal style flasks fitted with forged iron frames and cut off levers.
Yes, with a cut off valve the spout could be incorporated as a measure. This could be a very desirable feature especially if they were all cast to a uniform size to throw the specific charge needed for a standard military load etc. For non military use it seems the Queen Anne type flask evolved, maybe because it could be shop made from horn and needed no foundry made parts.
Finally, I dont think the pine wooden ends were actually needed and doubt they were bulk manufactured that way back in the day. Again, I'm really no expert and these are just all my thoughts.
TC