Also Need Help With This Probus Roman Bronze

On coins issued prior to the reign of Diocletian, the letters in the exergue aren't "mintmarks" in the conventional sense. The letters "XXI" are an indication of the silver fineness: 20 parts copper to 1 part silver. The remaining letter(s) are the officina control marks, which I believe in this case are actually "VI", sixth officina.

The reverse type is SALVS AVG, Salus seated feeding snake on altar. Sear attributes this type (# 12030, though with a different obverse type) to the Siscia mint. Found this example of the type on Wildwinds, which lists it as Cohen 591.

"I'm just surprised that the coin contains a bit of silver"

In the third and up to the mid fourth century, Roman bronze coinage almost always had some silver content. L. H. Cope argued that these coins should be called argentiferous bronze coins. The silver content fluctuated with various reforms, though.

From A.D. 307-317, the amount of silver was circa 2-5%, and generally eastern mints had higher percentages of silver than the western mints, until Constantine became sole ruler of the Empire. From A.D. 318-320, there was circa 4% silver in the coinage. From A.D. 321-330, the silver content was circa 2%. After A.D. 330, the silver content was circa 1%, which was maintained until A.D. 341, when the silver dropped to less than 0.5%. Coins with only a trace amount of silver can no longer be thought of as argentiferous.

The argentiferous alloys were comprised of mainly copper (Cu), lead (Pb), tin (Sn) and silver (Ag).

Just one example from a Constantine coin circa AD 310-- copper 87.46% tin 5.27% lead 4.88% silver 1.74%

There may have been multiple reasons for using lead, it did make a coin seem heavier, it was cheap, maybe it helped the alloy bond, but I believe that a practical purpose was also that lead made coins easier to strike and the alloys with more lead would also have yielded coins that were less brittle, and dies would have also lasted a bit longer.