Spent last night retrofitting a fancy electronic combination lock on a metal door for the first time, and encountered something I don't understand.
The door has previously had another mortise lock, so I assumed it was set up for the American Standard Mortise and this would just be a matter of drilling the additional holes to support the lock body. I was half right; it was ASM and the mortise body it without any trouble. There turned out to be a problem with different knob center and having to enlarge that hole from 3/4" to 1" and shift it upward a bit, but that was expected.
What I didn't expect was a battle drilling the mounting holes. Drill first hole, no problem. Drill second... and first is suddenly partly blocked by an internal piece. I had to continue arguing with that for the remaining four holes above and below the lock body.. If I'd known to expect that, I'd have brought a bunch of dowels or extra drill bits or something to try to stabilize the position of that... but I didn't expect it, and I'm still having trouble explaning it.
Question: Why would there be an apparently mobile sheet-metal inner box within the mortise opening? Is this a normal part of the design of these doors (and if so is it general or manufacturer-specific), or is it something a previous locksmith added to strengthen the door (seems unlikely), or was it something that should have been spot-welded in place but wasn't, or...?