500,
Yes, and there are (or were) a couple of others as well, here in Oz.
The rifle in question was supposed to be a 300 Sherwood, and well it may have been.
However, European doubles are often a little vague regarding the actual chambering and I would have been wanting to obtain a photo or details of all the barrel stampings/engravings before believing that the rifle truely was chambered for that cartridge.