Larson Storm Door Hardware - Mortise lock body Only
Faceplate: 13/16 wide x 7-3/4 Length
Backset: 7/8 inch - See additional Pictures
Cylinder: American type with Flat tail piece.
Note: Larson has another Door with same type of lock body, with 1-5/16 Backset
Please Review additional images and compare measurements.

Larson Related Storm Door Hardware-
305009 - 7/8 inch Backset
305010 - 1-5/16 inch backset 

Oil Rubbed Bronze finish is discontinued and no longer available. 

How to Replace or Change Out a Mortise Lock