The Wikipedia Money supply table here lists traveler's checks as a component of M1 money.
You see it at FRED, too:
M1 includes funds that are readily accessible for spending. M1 consists of: (1) currency outside the U.S. Treasury, Federal Reserve Banks, and the vaults of depository institutions; (2) traveler's checks of nonbank issuers; (3) demand deposits; and (4) other checkable deposits...
I finally got tired of seeing traveler's checks listed as a component, and not knowing how significant a component it is. So I interrupted myself and graphed the thing:
|Graph #1: Traveler's Checks as a Percent of M1 Money|