In my view they are both european Elm, but the top rifle has more heartwood in it.
Despite the info in the Bogdonovich book, where he states most M48's were stocked in beech, I've only come across maybe 2 M48 series rifles amongst hundreds that might have been beech. The very great majority were elm-stocked.
The first couple years of M48 production used a lot of walnut, which presumably the yugosavians imported from some place like turkey. These days, when you see walnut stocks, it's obvious as they used a very dark variety, and the stocks are most often re-numbered as the earliest guns saw lots of use and were often re-built. In this last batch from Tradex, I ran into a couple guns with walnut buttstocks, re-numbered to match and roughly sanded, with elm hand guards. Both were still in the arsenal storage cellophane and came out of war reserve storage like that.