This gentleman belongs to Reserve-Kavallerie-Abteilung Nr.76 (76. Res. Div.). As you can see from the document below, this was a wartime offshoot of either GDR 23 or LDR 24... Hessian dragoons anyway.
https://www.deutsche-digitale-bibliothek.de/item/YQZFXKNTDN2IMBBLYAWHWZHMHKY6QG7X" onclick="window.open(this.href);return false;
His appearance is consistent with what Jürgen and I know about similar Saxon formations - basically the uniform of the parent regiment is worn, but the helmet cover ('RKA 76'?) is something we've never seen before. Very interesting indeed!