Any ideas on whether this is authentic? It looks like a Texas Militia buckle from the front, but the back has a much larger tongue than the examples I've seen.
Thanks.
First... Texas was not the only state using a 5-point star in its "state emblem." Among the others, Maine and Mississippi used it, as shown by your star buckle having Mississippi stamped into its back.
Various body construction clues on your belt-plate indicate it is not a reproduction.
It is what is known as a Militia "panel" beltplate. See the detailed info about that type of plate on page 250 in the book "American Military Belt Plates" by O'Donnell & Campbell. It is called a "panel" beltplate because the emblem is shown inside a scalloped-corners "panel" at the plate's center, with the panel being surrounded by various decorations.
Militia "panel" plates made before the 1870s were thin stamped sheetbrass, showing a reverse-image of the front's emblem on its back. The version you found is solid-cast brass, which is thicker than stamped sheetbrass, and does not have a reverse-image on the back. So, your Militia panel plate dates from the latter-1800s into the early-1900s. The "American Military Belt Plates" book shows your EXACT plate on page 250 and says it appears in the 1877 catalog published by Horstmann Brothers & Company of Philadelphia PA, a major dealer of Military, Fraternal, and Theatrical beltplates and regalia.