I own a matched pair of Boswell boxlocks, 13,459 & 14,412 ( the second gun was made to match the first by Boswell). They were delivered in September 1899 and 1902 respectively.

So I would suspect your gun is circa 1896-97. Even though you have thick barrels I suggest keeping your pressures below 8,000 psi to protect the action and stock. RST and B&P make great low pressure pheasant loads.

Enjoy!
Dr. P