mortise lock inquiry

The mortise lock on my screen door requires replacement. Can you please assist me in determining the specific part needed? Attached is a photo of the lock, measuring 4 3/4 inches in height and 3/4 of an inch in width. The distance between screw holes is approximately 3.5 inches, however the lock appears to have an outer shroud that fits into the door. It is important that the replacement lock fits the existing hole in the door, otherwise it will not sit properly. Thank you