It looks like
RIC 519a, not 519b which is the corresponding middle bronze.
It took Frans a long time to acquire such a
sestertius, but it is nevertheless not a great
rarity. A
fair number of specimens and different dies are known.
I believe I once gave Frans a count of how many specimens and dies of
sestertii of the different issues of 138 are in my
collection of plaster casts, but I can no longer find it via a
Forvm search.
Anyway, specialized
rarity has virtually no effect on the market value of an ancient coin, because there are so many specialized
rarities of
ancient coins, and because there are very few specialized collectors who are on the lookout for
rare varieties. The usual collector of ancients is a generalist who wants nice coins and historically or artistically interesting
types, but doesn't care about
rare legend varieties.
So it's the nice condition and
fine portrait that will determine the market value of your coin, not the somewhat
rare legend variety!