Mike, the pics show all the markings. Only other thing is the rest of the serial number.
From the third volume of Nigel Brown’s British Gunmakers, the length of the serial number might add information. J & W Tolley used a four digit serial number into the beginning of the 20th century. Holloway & Naughton succeded to the business after 1911, and after the business was acquired by Skimin & Wood in 1929, a five-digit number was used, a continuation of the S&W numbering system, until 1964.
Canvasback, if yours has the four-digit number, it appears to fall within the 1875-87 range, and presumably the nitro proofing came later.