Yup, it is very likely the museums all
had examples. In fact, for the Emperors that ruled for very short periods, I find museums to have over-focused on obtaining examples, and in some cases are missing far less
rare coins from longer-ruling Emperors. The curators
had their own biases and likely valued getting coins they thought museum-goers might not get to otherwise see. That has little to do, in my opinion, with actual
rarity in collecting circles today.
I've always found
hoard finds to be more indicative of relative scarcity than what
Mattingly thought in the 1930's.