W&F Style mortise lock - round end with 45 degree slot when locked. 

Available Finishes: Zinc Dichromate and Stainless Steel

7/8'' Backset
Installed with 6-32 machine screw 
Internal Latch for primarily Wood and Vinyl Glass Sliding Patio Doors 

Note: Metal Doors usually use the 15143 vertical keyway Latch

Hole Center: 3-11/16-inch
Keyway: 45 Deg.

VIDEO: Replacing a Single Hook Lock in a Sliding Door