I have stumbled across many depictions of Roman coins with the abbreviation CONOB on it, such as this one.
What does it stand for?
According to a poorly cited Wikipedia article CON indicates the mint of Constantinople and OB is an abbreviation of obryzum, and quite literally translates to "1/72 of a pound of pure gold".
CONOB is a legend found in much of Byzantine gold coinage . Sometimes COMOB is found.
It was found, for example, in exergue in the solids produced by the mint of Constantinople.
CON indicates the mint of Constantinople.
OB has a double meaning:
the two letters in the Greek numbering system correspond to the numbers 72 and indicate that the monetary foot used for the solid is 1/72 of a pound they are also the beginning of the Latin word obryzum , 2 which indicates refined, pure gold. So OB in this case means 1/72 of a pound of pure gold.
FORVM ANCIENT COINS
For a non Wikipedia reference you can visit FORVM ANCIENT COINS. It says the same thing.
It appears your answer is here:
CONOB. Constantinopoli obryzum. The solidus weighed 1/72 of the Roman pound. "OB" was both an abbreviation for the word obryzum, which means refined or pure gold, and is the Greek numeral 72. Thus the exergue CONOB coin may be read "Constantinople, 1/72 pound pure gold." -- "Byzantine Coinage" by Philip Grierson
Just to partially confirm from other sources:
Here it says CONOB a mint mark indicating it was coined in Constantinople. Although, given the meaning given by the first reference, I am not sure if CONOB would also be used for other mints outside of Constantinople but inside the Empire and under the authority of the Emperor - it may only mean that it is an imperial coin. Also be aware that if crosses are defaced or eliminated, your coin may be an Arab imitation.
And CONOB is called 'mintmark' here: https://www.wildwinds.com/coins/byz/heraclius/i.html