Your reference is sound, as suspected it is copied from the PA CBU book, also known as the "Blue Book". I am always frustrated by these sites that pirate the work from other sources, without giving credit where it is due (see Gordon's thread). Without giving the source you prevent researchers from finding additional information that you failed to include, source data, etc. Whether it is intentional, lazy or what, I don't know, but it seems to be happening more and more often.
based on the photo provided, I still doubt that this is an original gravel mine. The correct material is a tighter weave, and a soft cloth. I have a number of examples, several of which were purchased from the estate of a former worker at the production facility. I have also had the opportunity to examine and photograph the identification boards which show several of the models, at the former ordnance collection/museum at Picatinny Arsenal. They just are not the same.
A number of copies have been made over the years for legitimate purposes. I have seen them in E. German mine identification sets, Chinese ID sets, and US training materials as well. These have their own collection value as training items, but should be identified correctly.