Just for info, the chubb is used to measure bore diameter from the breach end to determine wear, it can also be used to measure chokes. Yours is in great condition and will measure 12/16/20 bore. I paid £240 for min about 3 years ago from E Bay
It has setting rings for 12, 16 and 20 bore. It's not just for measuring chokes, it will measure the bore diameter as well. It can, of course, be used to very precisely determine the degree of choke unlike the so-called choke gauges you put in the muzzle. That looks like a fairly recent piece of kit (setting rings are metric by the look of it) and in very tidy order. It's worth a heck of a lot more than £65.00!!!!!! Look at the results for these things in the Holt's sales - you will be surprised.