OK. Fender loaned me a front coupler (Thanks, Dan!) so I can defer the decision about which coupler to buy. Now another problem...
Everything I read here and other places states that the coupler center height for 7.5" rail should be at 1.5" scale, or 4-5/16" (4.313) above the rail (34.5 divided by 8 ). My P7 is a 1.6" scale--divide by 7.5333 to be exact, which would set the height at 4.580 inches. I have a scaled prototype coupler pocket, and it looks like the one in the attached photo of the President Washington, the B&O P7 I am building to. This pocket is a standard B&O part that was used on many of their locomotives, so you will notice in the photo that it is mounted low on the bumper, and the three attachment studs on each side do not use the standard hole boss locations. It's like that for real and in my prototype blueprints. Well, the difference between 1.5" scale height and 1.6" scale height comes out to 0.267 inches, which puts the pocket wayyyyy down on the bumper. Picture the distance, in the photo, between the top of the coupler and the top of the bumper as 0.300, and where it needs to be for 1.5" scale at 0.567, or nearly double, and you will see what my problem is (or maybe you don't see a problem).
Photos I have of one of Bruce Hamilton's P7s show the coupler at at or around 0.300, so either the standards were different for 1.6 scale (or didn't exist) in the 1970s when he built it, or he just put the coupler at the wrong height. And I can see in photos of his locos that he didn't alter the pocket to lower the coupler. In any case, both of his locomotives have been running like that for nearly 40 years, and I haven't heard anyone complain.
So how much can I fudge the height of the front coupler without having a problem? Half the difference (.130 or so)? How precisely are the couplers lined up on average out there in the live steam world (I don't get out much to see for myself
). I want to get this where I need to before I start drilling holes, which I was planning to do today. BTW, I'm measuring at running weight, and it's sitting at design height. I don't expect my springs to sag a lot but if they do, I can run the coupler up in the pocket. I can't run it down.