Surely you have looked at the proof marks on the barrel flats ? It will state the calibre etc there and the proof house stamp -- probably Birmingham for a Tolley
I am not new to reading markings of British Double rifles. The barrel rib stamped "J & W Tolley, 1 Conduit Street, Regent Street, London". Therefore the year of manufacture would be 1883-1893. In 1883 the firm opened a shop in London at 1 Conduit Street, in 1894 this moved to 59 (New) Bond Street. There are Birmingham Proof stamps on the barrel flats and the action table. There are bore markings of "39" on the bottom of each barrel indicating a bore of .492". The barrel short rib is stamped "J&W.T" - "STEEL" - "6732". But there are no caliber markings. The side of the action is marked CHARGE 5 DRAMS, but that does not imply caliber.