Without being an "advanced collector", I think the answer is quite simple: these stock disks are no longer a guarantee of anything! They have been stripped from scrap stocks and refitted in other rifles to give a "better" rating, and even copied. I have seen a tobacco-tin full at a sale. Even if a disk is originaly to a given rifle, it only indicates what the state of the barrel was when the rifle was inspected in active service, and that can now be the best part of a century ago! Heaven knows what hands it has been through since. Buffdog, I agree with you - that disk is not original to that rifle. The disk has been retrofitted in a cut-out that was too large for any stock disk. In other words, my guess is that at one time a previous owner installed a coin or medallion in place of the original disk, and that this has since in its turn been replaced by a stock disk from the spares box. One sure-fire way to tell that such a component is not original, is to examine it under a watchmaker's eyeglass. There wil be a mismatch between scratches and wear on the metal compared with the wood.
The full lowdown on stock disks can be found in "The Crown Jewels" by Dana Jones, P 159-169. 10 full pages with photos which I shall not attempt to repeat here!
Patrick