The description says "Republican belt found in a military camp in Numantia, Spain, mid 2.century B.C. to 1st century B.C."
I've asked them about the source of the plate design, but perhaps they are on holiday in a warm location. My searches so far have turned up nothing that looks like the plate in question, but I know Erik and Ira are well regarded for their research and I'm impatient to know.
No pics of said belt plates, unfortunately, but in "[hide]Of Belts and Men: The Roman Military Belt of the 1st Century A.D.[/hide]" Andrew Magee claims: "In addition to openwork plates, solid plates also come from [Numantia]. Some use only incised lines in simple hash mark designs [...] (p. 148)."
Regarding Numantia, Magee lists the following to sources:
They may have a reputation of making good reproductions but I look at some of their object and often wonder where the evidence is. Overall, I'd say he is a good smith but in my opinion far from great. I sometimes doubt what he writes. Just my two cents.......
"You have to laugh at life or else what are you going to laugh at?" (Joseph Rosen)
You will find a plate of this type attached to the round breastplate from Numantia.
See Mr. Bishop's "Roman Military Equipment" p. 64 fig. 31.
In the book by M. Luik, "Die Funde aus den römischen Lagern um Numantia im Römisch-Germanischen Zentralmuseum" (Mainz 2002) there is a second plate now attached to the top of the breastplate and an identical third piece which is catalogued as a possible beltplate.