Steel keeper length is 6"

I'm afraid I can't say for sure what keeper will work based on the mortise lock. Your particular mortise lock can work with many, many different keepers, including the 89-021. If you don't have the original handy, it'll be hard to say what can work. To start, can you show us a picture of the space where the original keeper installed? Can you also measure the distance between its old installation holes and tell me what you get? Make sure to measure from the center of each hole, not their edges. Thanks!