I've looked at quite a few pics of other T's and it seems some have this hole left open, some it looks partially open, while others seem to have something like a lock cyl or device of some sort in it.
When I took the door apart, there was a set screw on the inside of the jam that looks like it was meant to hold something in there.
Ha, forgot to post the pic.......
Hole is for a door lock.
What year? Was that factory or aftermarket?
It's a 1927 coupe, so there is a lock cyl that should go there. Hmmmmm Strange that they placed it on the pass side.
Or maybe it wasn't for security, maybe to keep the pass door locked when not in use?
The set screw holds the lock cylinder in place. This happens on the passenger door and the drivers door is locked form inside by pushing the latch lever forward instead of backward.
That is how my '25 Tudor is.
Best I remember the set screw holds the lock and it is on right side because it is hard to enter on the left, and the right is on the side of the curb so you do not get run over when parking and locking the door. (That is the explanation I have heard)
Outside locks were on the passenger's door on lots of early cars. My 47 Chev truck is this way. I was told, like Tim, that it was a safety thing.
I hadn't thought about it, but my '53 Dodge pickup is that way too--and, yes, what Tim reported is what I have been told for decades. I wonder if the door locks in England are on the other side?
The set screw is a headless slotted screw with a fair point on it (usually--but not always!) The lock cylinder has a tapered flat spot on the side for the screw to clamp on.