I believe it may be three $\mathrm{M}$’s with two bars on top but would like confirmation. I am also being told it could be $\mathrm{CD}$ with four bars on top.
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$\begingroup$According to Wikipedia, several methods toexpress numbers $\gg 1000$ were developed:
Apostrophus
This would write $10^{14}$ as $${\text{CCCCCCCCCCCC}}_{}|\text{ƆƆƆƆƆƆƆƆƆƆƆƆƆƆ}$$ $5\cdot 10^{14}$ as $$|\text{ƆƆƆƆƆƆƆƆƆƆƆƆƆƆƆ}$$ and so $(5-1)\cdot 10^{14}$ possibly as $${\text{CCCCCCCCCCCC}}_{}|\text{ƆƆƆƆƆƆƆƆƆƆƆƆƆƆ}|\text{ƆƆƆƆƆƆƆƆƆƆƆƆƆƆƆ} $$ Ugh!
Vinculum
(Julius Caesar would not have used this method.) Here, we'd start with $400$ (i.e., CD) and multiply by thousand three times, so $$ \overline{\overline{\overline{\text{CD}}}}$$ In late-medieval variants, also things like $$ ||\overline{\overline{\overline{\text{IV}}}}||$$ or $$ ||{\overline{\overline{\text{MM}}}}||$$ might be possible.
$\endgroup$ 1 $\begingroup$According to this site, the Romans did not originally use numbers large enough for this to be a concern. Just going to 3999 was enough. Later the use of the viniculum or brackets to indicate multiplication by 1000 was added to accommodate changing needs, but no standard practice was established until later times and later users.
So I would argue that the trillions in 400 trillion could be rendered however you want!
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