I would agree with HistoricBore that it could well be the original pad that has lost its shape. I have just found that the Silver’s pad I fitted about 40 years ago to one of my rifles had collapsed under the weight of the rifle stood butt down in the cabinet. Fortunately I had a suitable replacement.
This reminds me of the days, probably now the best part of 50 years ago, when William Evans still kept rifles for sale in a rack backing on to the side window of the shop (we were less security conscious in those days).
I was particularly taken by a brace of cute little 6.5 MS carbines. As they were too short to stand safely in the rack by themselves they each had a Parker-Hale cleaning rod inserted into the muzzle to hold them in place.