The proper Dave Brown for the OS 25 is the DB 2025 not the DB1519.

Dave Brown mounts all have a tapered bottom surface to the beams, You can drill through and use nuts however unless you grind flats where the nuts go the bolt will break every time at some point. This is a very poor makeshift alternative.

Using them as they are intended they are superb and I use nothing else because of the round back face and the non square firewall bolt pattern they can be fit in tighter places than any other mount.

Your mistake was in drilling and tapping the beam holes, will strip every time. Use only the indicated size pilot holes indicated on the back of the packaging. The proper pilot drill size for the 2025 mount is 3/32, use no other size. Now the supplied screws are driven in (Do not tap). The proper size and supplied screws for the 2025 is #4 x 1/2 inch sheet metal screw.

I would reccomend you get a new mount and use it as intended anything else will only come back to bite you.

John