So, nearly a half pound more for your Model M. That is quite a bit. I suppose the steel buttplate, steel forend cap and the peep sight could account for the difference. Also, a DST might weigh a bit more than the single trigger. And differing wood densities can add or subtract a few ounces as well. I think that without using the same scale and breaking down both a Model M and a Model S and weighing all the components, there is really no way to do this any more accurately.

Thanks