All of the french doors (OK, both sets of french doors) in my 1922 house either have or had slide bolts that bolted into the floor or the upper doorframe. There was no little loop of metal, but a kind of metal cup set into the floor or doorframe to house the bolt. I like the way that looks.

One door already has slide bolts inset into the door edge at the top and bottom; with the slide bolts engaged this keeps one door closed so that the other door can latch to it.

However, I need a LOCKING solution. The doors open inward into the room, I need a solution to lock the doors from outside the room to prevent entry. Not just a latched-closed solution but a LOCKING solution.

Here's another pic, showing the door edge profile:

Historic House Parts has some interesting-looking deadbolts.

Here is a link that might be useful: HHP deadbolts

Call Brad at Charleston Hardware.

Here is a link that might be useful: Charleston Hardware

Those doors are thicker than 3/4"

jumpilot, I bet it's not thicker, it's just an illusion from the thickness of the astragal.

There are locking slide bolts you could install on the desired side, but they appear to be intended for non-wooden patio doors and are rather hideous - maybe with some luck you can scare up a nicer version?

There may also be locking Cremone bolts, although this might mean re-doing the whole knobset - hunt around.

Here is a link that might be useful: locking slide bolt

1-3/4" is what the op meant i am certain.

My interior french doors have a rabbeted mortise lock set similar to this

This one doesn't have the matching strike plate. I'm just using it as an example. You'll also need the knob and face plate

below is a repro one from house of antique hardware

Here is a link that might be useful: Rabbeted lock set

Thanks a bunch for all the good input.

1)The hardware links upthread are great, although I haven't located anything from them that fits the bill.

2) Jumpilotmdm & millworkman:
You're right. Doors are 1-3/4" thick.
Typo corrected.

Btw: Doorknob setback is 2-1/2" - 2-5/8".
(I took off a touch too much wood on the door edge to get the doors to fit

3) circuspeanut:
'There are locking slide bolts you could install on the desired side, but they appear to be intended for non-wooden patio doors and are rather hideous - maybe with some luck you can scare up a nicer version?'

It's concept that had occurred to me (even though the doors already have two slide bolts on them), but I looked around and couldn't find a locking one. And your right regarding hideousness ... might as well put a hasp and padlock on the door, which I am unfortunately very close to doing as my time is running out and I need a solution.

4) sacto_diane:
A rabbeted mortice looks promising, though it seems it would be perhaps a federal case to get locate one that would fit? Also, I'd prefer if at all possible, a modern key vs. skeleton key, but if I could locate a correct rabbeted mortice I'd likely call it a day.

never mind, incorrect thinking on a Monday AM.

This post was edited by millworkman on Mon, Sep 9, 13 at 9:01

Another solution is to use a transom bolt which locks the inactive door until the active door is opened and a regular lockset on the active door

https://www.accuratelockandhardware.com/product-page/04-transom-bolt