The Type 1R Small Latch Right Hand and the Short "P" Receiver SPR250L-1 are normally surface mounted in the same plane - no mortising required. They can be mounted externally or internally. 

Fig1. Mounting Dimensions
Type 1L Small Left Hand Latch: S1125-1R-562
Short "P" Receiver: SPR250L-1

 When internally mounted, the latch is operated through a key access hole. For Sealed untis see TDS 25-1 
This Short "P" Receiver is being replaced by the newer SPR250 (L&R) - See TDS 19-3A and 19-3B