The dimensions of the case are 3 1/2" x 4 3/4" so based on your description and with the way the elastic band is oriented, I think this would be a cigarette case. I think the presence of the elastic band would also indicate the piece was made somewhat more recently?
I went to a second shop thinking they had a handheld XRF that could analyze my larger pieces which didn't fit in the bench top XRF at the first shop (unfortunately there was a miscommunication and it turned out they had the same bench top model as the first shop). The second shop measured this again and it came in slightly higher at 88.1% Ag with the balance being Cu and 0.1% Pb. I am wondering if the difference was due to calibration, or if the surface is not perfectly homogeneous.
I zoomed in on one of the shaded areas of the horse, and the shading effect seems to be accomplished by a fine scoring of the surface. The grooves of the shaded areas retain some oxidation which contrasts with the higher smoother surfaces and the heavy oxidation of the background. The shading/contrast is more visible when the light is reflected at certain angles (such as in the first post).
For having such fine detail on the exterior, the interior is noticeably less refined and is quite rough near the perimeter of the case. As for markings, there doesn't appear to be an "84" marked anywhere, but there does appear to be traces of an inscription in one of the corners (although it also could be a series of random gouges that have the appearance of script?).