Just a thought - a signet ring is usually oval in shape and, when completed by the engraver, an impression such as yours is supplied with it to confirm patter, depth of imprint, etc.
A desk seal is more round, or even square, and I suspect this is what you have here. An imprint of a desk seal - they stood about 2 or so inches high, basically a seal with a handle attached but fairly elaborate.
A ring tended not to be square because the overhang of the circle on the finger would get in the way, hence they were made oval.
Regards
AJ