I’ve learned the hard way a few times. When it comes to Native American made jewelry, if you see something you like, you should buy it because it is likely you might never see the same thing again – this is especially true of vintage items.
Overlay pieces are made of two layers. The bottom sterling silver layer is a solid piece. The top sterling silver layer has a design of a scene, figures or symbols meticulously cut out and then placed over the bottom layer. The two pieces are “sweated” together, that is heated, so that they become one. The bottom layer is the background behind the cutouts and is traditionally darkened (oxidized) for contrast. The result is a 3-D picture with great depth and interest.
The bottom oxidized layer (background to the cutout) might be smooth or accented with hash marks. The Navajo silversmiths usually leave the oxidized background smooth like the Sunface pendant by Charlton Lindsay shown below.
Hopi silversmiths etch the background (texturize it) with hashmarks before oxidizing it, like the kokopelli maze buckle by Joe Josytewa shown at left.
As far as the hallmark on the buckle you missed out on, according to Hopi Silver by Margaret Nickelson Wright, variations of the sunface hallmark are associated with a number of independent Hopi artists as well as a quite a few members of the Hopi Silvercraft Guild. The Guild is an association located at the Second Mesa which began producing silverwork in 1949 and continues to the present. Each smith has his own guild stamp, and although all look similar since they are sunfaces, no two are alike. There are a number of examples in Hopi Silver and none look exactly like the one on the buckle you wrote about.
As far as finding something similar, we have a number of outstanding Hopi buckles on this page.
PS. An observant reader pointed out that the buckle depicted a bird when flipped over. She saw an Thunderbird. I see an eagle from the Hopi Eagle Clan.