I am sure that you know that your gun was proofed under the British proof system. This link may help you if you don't: http://www.basc.org.uk/media/100_proof.pdf

Essentially your gun has pre-1925 Proof Marks. These DO NOT give the chamber length as, at that time, a shooter would be assumed to know from the shot weight - 1 ounce - what they were.

Please be aware that your 16 is chambered for 2 1/2" 16 bore shells and NOT 2 3/4" 16 bore shells which would have been marked as 1 1/8 ounce, I believe, from memory. Therefore even though the service load is one ounce you should not use 2 3/4" or 70mm "shells" regardless of their shot load weight!

The old Eley Alphamax 16 bore in a 2 3/4" case fired the 1 1/18 ounce load, the still current Eley Grand Prix 2 1/2" case fired the ounce load that is the service load your Watson was proof marked for.