Keeping lock from dropping into door frame

A quick learner from Riverside, CA says:
What is the trick to not dropping the lock in the door frame during installation?
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Tom from SWISCO responded:
Thanks for posting. Your mortise lock is dropping inside your door frame? That probably shouldn't happen; this lock has a faceplate that is wider than the hole. It's possible that your have the wrong lock. If you wish, you can send a few pictures of your lock as compared with the installation slot for us to take a look at.
A home owner from St. Louis, Mo says:
I have a 5500 series sliding glass door, the mortise lick I received is slightly larger than the opening in the door frame. I don't think this is a problem. I can file the door frame opening for the lock slightly and it will fit. I don't remember how the latch moved to the locking and unlocking position, there is no thumb handle to operate the locking lever. It looks like the lock is 55-537 as noted on the shipping order. Please advise about the locking device.
order No. 20079399 customer no. 3452
A home owner from NJ says:
I have the same problem here. The 3-11/16 is definitely the right size but for some reason the whole itself is about 4" in size. On my door it appears thee are holes for some kind of face plat to keep the lock in place. Picture attached.

In theory, the faceplate is not needed because as soon as you install the handle into the lock it keeps it in place. It should only be an issue if you remove the handle and the lock falls into the door (which happened to us).
A customer submitted photo of a mortise lock.
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Donna from SWISCO responded:
Looking at the image you posted with the ruler, it looks like the installation holes might measure 4-5/8" on centers, if they do please take a look at SWISCO product code 82-226, mortise lock with faceplate, as a possible replacement.
A quick learner from St. Louis, Mo. says:
Got the lock installed after a little filing of the opening, but it works great now.I was able to get a long nail thru the locking handle hole and the door jamb to hold the lock in place untill I got the screws installed.
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