That altar from Ostia looks more like a hole with a wreath carved around it, like it was meant to hold something, like scrolls. Maybe a PO box to the gods.
Yes, or maybe just a cubbie-hole to store the booze.
So, after doing a little research, I've got a better considered idea...
The "dot in circle" on the VLPP altar may be a libation bowl (a patera without a handle) used to make offerings. The "dot" would be the raised (hollow on reverse) center of the bowl, that was apparently there to make it easier to hold.
Here's one from 1st-2ndC AD sold by HJB where the raised center is a decorative frog, and another much earlier plain one from 7thC BC in the Met museum.
And here's a Nero aureus (RIC 76, from BM) showing how this "dot in circle" was used to depict one of these.
So, apparently altars were sometimes decorated to depict sacrificial implements, such as this 3rd C AD one from Niederbreg castle:
And here's a remarkably similar patera and knife in the BM:
And finally a 2ndC AD altar in the Ashmolean museum with a libation jug on one side and a libation plate/bowl on the other: