entry door backset

I have run across a couple of doors that had unusual backsets during my years as a GC. One of them had door knobs in the center of the door. Each time that I had to deal with these, upon taking them apart I have found that they had what seemed to be custom manufactured "extenders" for the bolt. I was able to replace them with standard door hardware and reuse the extension bolt. You might take yours apart and see if you can do the same.

=== Try here:

That sure is a bastard size. I've seen those you mentioned, and I've seen 5" and 18". You have my sympathies.

I've seen Backset extensions for some locksets at Locksmith shops.

There's much more available in specific trade shops/retailers than you will find in any of the big stores.

LdB