I have a 1961 Model 100 in 308, serial about 20,000, which has been fired 3 times since it left the factory. Unfortunately, it is missing the buttplate. Question: Does anyone know for sure what sort of buttplate it used, and by chance does anyone have one which they would part with?

Your gun was originally equipped with the black, hard rubber plate. M70 and M88 plates are the same....you may have to do some fitting. Try eBay....