I just checked the Buffalo Arms site. They have both the Jamison 25-20 Single Shot and the 22-3000 brass formed from it in stock. The site says "ample supply".
I recommend the C.S. Landis books "22 Caliber Varmint Rifles" and "Woodchucks and Woodchuck Rifles" as potential sources of information on such rifles. A chamber cast & careful measuring of groove diameter & twist is recommended. Back in the day, 22 barrels came from many sources, often from rimfires.
The combination of a DST that must be set for the rifle to fire and the absence of a safety indicates that it was carried afield empty or loaded but with the bolt handle raised. Treat it gently when loaded.
