Michael, when you infer a date then the date in the call number will also be that year without an embellishment.  See https://www.loc.gov/aba/publications/FreeCSM/G140.pdf on how to enter dates in the call number.  Also fixed field should be Dtst = s and use only the first date box with the 2018 in it.
Don’t forget to include a 500 note that tell the source of your date. Eg.  500 _ _ Date inferred from “12/18” or whatever the numeric code is on your piece.

From: Maps-L: Map Librarians, etc. <[log in to unmask]> On Behalf Of Michael Fry
Sent: Friday, March 17, 2023 2:34 PM
To: [log in to unmask]
Subject: inferred dates and RDA

When cataloging resources that lack explicitly stated info (e.g., statement of authority, date of publication), my understanding is that one can record inferred info in square brackets. So in the classic case of CIA maps--which typically imply through a somewhat opaque alphanumeric code when the maps were created--the 264$c might look like this:

[2018?]

But in that case, what about the 008 and the 050/090 fields? Is it equally permissible to put "[2018?]" into those fields? I've seen plenty of CIA maps, but I'm not sure I've ever seen one with a "?" in the call number!

Thanks.
Michael