The standard Desert Tech thread pitch was 3/4x24 on the A1 and older barrels. They didn’t start offering the 5/8x24 for smaller calibers until the SRS A2 came out.
I have 2 new in the packs Thunderbeast 338 Muzzle Brakes 3/4x24 threads. Fits 338 Ultra Suppressor. $125 shipped each. PayPal f&f or you pay the fees, Venmo and Zelle.