It's not listed, but R dot P was used a few times at Rome for different issues (see #79, 184, 189, 225).
On many coins I have seen stray dots that I believe were used by the engraver, much like a centering dot; but yours does seem more purposeful, except if it was intended to be part of the mintmark, I would expect a fatter dot (second picture below is #184 to show the size of dot)- but have seen small dots before. I have attached a picture of RIC 272 from Wildwinds that looks like it may have a dot also, but it is not very clear.
So, I am not sure on your coin. I would call it a (possible) variation, but with the possibility of the dot just being some remnant of the engraving process or a product of the minting process.
I just recently added a picture of a campgate with a dot that shouldn't be there-
http://www.lateromanbronzecoinforum.com/index.php?topic=317.msg715;topicseen#msg715