This was in the Honey Island Swamp, right on the border of Louisiana and Mississippi in a bottomland hardwood forest. I collected it, and could send it to you if you are interested.

Are there other definitive characters that would resolve the species for certain? I'd be happy to take some photos at higher magnification, if that's necessary.

Also, is there a particular reference that you'd recommend for identifying Mutillids?

Here's the original species account by Charles Blake, p.249, Transactions of the American Entomological Society, vol vii (1879). Not sure if it's helpful but I was pleased to be able to find it, and the descriptive language is quite interesting.

"Mutilla euterpe. — female - Ferruginous; head round, finely and closely punctured, face deeply excavate at insertion of antennae; eyes ovate, finely reticulate; antennae fuscous, flagellum pruinose, scape finely punctured; mandibles dark ferruginous before the tips; thorax elongate-quadrate, emarginate at sides; pro- and metathorax rounded, the former scarcely as wide as the latter, closely and deeply punctured; metathorax abruptly truncate, stained with fuscous at apex; flanks of mesothorax smooth; legs black, thinly clothed with pale pubescence, intermediate and posterior tibiae feebly spined, calcaria white ; abdomen ovate, basal segment black, second minutely punctured, apical margin broadly black, wider at middle on disc, bearing a short longitudinal line of pale golden pubescence, apical margin fringed with similar pubescence, third segment black, apical segments fusco-ferruginous, fringed with hairs of the same color. Length 12 mm.

Hab.—Florida. This species is easily recognized by the form of the thorax; it is allied to fidicala Sin., which however has the abdomen longitudinally rugose."

That's great to see a pdf of Blake's paper. I think that the view we have of the top of the thorax is clear enough to keep the specimen identified as T. euterpe.

That's what I suspected for the genus, though I wasn't positive.