To be exact a 1.6 stiller TAC Driver (tac408) with a 585 bolt head.
Did you ask McRee whose mag they designed their DBM to fit? The McRee will certainly know.
As best I know, there are only four US companies that offer CT mags:
Alpha (
https://alphaindmfg.com/store/index.php?route=product/category&path=17) - single stack, 7 rounds, so a possible;
Cadex - double stack, thus no match;
CT (
https://cheytac.com/product/m200-m300-magazines-2/) - single stack, 7 rounds, so a possible; and
XLR (actual product produced by a 3rd party, possibly Alpha) - single stack, 7 rounds, so a possible.
From Alpha's website, and e-mail or voice-mail-only contact info, I have no idea whether they are even still in business...but you could at least leave a voice message and/or send an e-mail. And you could call CT but I suspect their DBM is proprietary - probably have to buy one of their mags to see if it would fit. Actually, could be an Alpha-sourced mag - which still does not mean it would fit the McRee DBM. As far as XLR, my guess would be that Alpha made the mag they offer.
Have you actually tried any of the above products? Tried to reach Alpha, CT, or XLR?
When you find a mag that works properly, please let McRee know so they can source some for their builds.