If your trust itself is called the ABC Trust, then yes, you engrave "ABC Trust".
But if your Trust is the Albert Beuford Cook Revokable Living Trust, engraving "ABC RLT" or even "ABC Trust" does not meet federal firearms identification requirements, unless the maker has an approved, signed letter of variance from BATFE.
Markings are a means of identication of the maker, and federal marking regs only allow
"recognized" abbreviations -- that means abbreviations that everyone will immediately understand. There is no need to spell out "New York" when everyone recognizes "NY"; similarly, "Corp" can be used for "Corporation" and "Inc" for Incorporated.
"RLT" is not a term in common usage -- walk up to 100 people on the street and ask them what RLT means, and 99 out of 100 times, they will say it's something that goes good with fries. Therefore, it does not qualify as a "recognized" abbreviation.
OTOH, "trust" is a recognized term, and there is no relevance to the recognition process of whether the trust revokable or unrevokable, living or dead. Thus, an NFA firearm made by a Revokable Living Trust may be engraved as "trust" but not as "RLT."
Now, if your trust is so famous on a national scope that when you ask people
anywhere what or who "ABC" is, they will say it's Albert Beuford Cook, then it is a recognized abbreviation that meets federal regs. Otherwise, you need to spell it out. AB Cook is acceptable, because they can go to the state and city designated by the engraving, and start looking for someone named Cook whose first name starts with an A.
But "ABC" alone does not meet the marking regulation, because it is mandated for identification purposes, and as an unrecognized abbreviation, it does not meet the identification goal.
Having been dealing with NFA regs for decades, I'm not new to this confusing mess. And the above is my painfully educated opinion.