In them thar days long ago, proofs did not have cameo frosting or polished fields.
A proof was struck from the best effort with which a die could be prepared. The proofs were the first coins struck from that die. Sometimes they were double struck, to bring up all of the detail. Other than that, they were the same as business strikes.
The coin pictured here has obviously seen some circulation, and now it is impossible to tell if it was ever a proof or not.
I have seen some business strike coins that have some element of mirror surfacing in the fields, but I think they have had the good fortune to have been struck from unworn dies, that have been well made. This situation makes the detection of some 19th century proofs difficult.
I suspect that DMPL Morgans are a different case again.