Aren't you guys forgetting horn? Lots of guns of old sported horn plates.

Also, Goodyear discovered vulcanization in 1845. I wouldn't be surprised to find out that rubber buttpads had kicked around for quite some time before Silver patented his pad. Wouldn't be surprised to find out that leather had been used to soften the blow, either. I've made a couple leather buttplate/pads myself. Sometimes merely rounding off the edges takes a big amount of sting out of recoil and a leather cover to a steel buttplate can do that nicely.