You have to be careful with M1Ds as repros and fakes abound. There are now repros for all M1D parts - bases, mounts, scopes, flash-hiders, and cheekpieces, everything except the unique M1D barrels which were made by Springfiels Armory, and which have the drawing # D7312555 stamped on them. Early barrels had this number stamped on top of the barrel below the handguard; later ones had it stamped on the side of the barrel. Early Springfield assembled M1Ds will show an "open box" cartouche with a single letter on the left side of the stock, but stocks did get changed out in service, so replacements will not necessarily show this marking.
The other thing about the M1Ds is that none were originally manufactured as new rifles. They are all arsenal conversions of existing standard rifles, so they will be found built on Springfield and Winchester receivers, and to a lesser extent on IHC and HRA receivers. Some claim the Danes may even have cobbled together a few on their Beretta and Breda receivers. This is what lends them so readily to fakery. Also, because an M1D is by definition a rebuilt rifle which could incorporate parts from any US maker on any US GI receiver, there is no such thing as a "standard matching" M1D.
If you can rule in the unique barrel and rule out fake scopes/mounts etc when examining a rifle, you could consider it as authentic as the next one.