>Any idea of the date of manufacture?
None at all, except that due to the corrosion in the bore and under the relatively new paint, it has some age. I'm guessing it could have gotten that kind of corrosion from storage in a basement for 100 years, especially if it was unpainted during that time.
I agree that it looks like some kind of 19th C. generic Armstrong gun.
One little clue to how this item has been used is the vent, which is fully large enough for firing, and appears to have been fired due to some wear. An "arsenal" model made strictly to illustrate a full-size gun of the same design but larger dimensions would have the vent made to scale, which means it would be far too small for firing with either fuse or friction primer. We have some arsenal models and the vents are only large enough for a pin to go in.
Since the bore surface is more corroded (pitted, uneven) than the outside surface, I suspect it means the gun had been fired with black powder and not cleaned. The powder residue attracted moisture and formed acids and chewed up the metal more than on the outside.