I think I got the 51r, but it's all water under the bridge now. I took it back to WallyWorld, went to the Honda dealer and got one that fits like I should have done in the first place (and it was $20 cheaper). Not a lot of time wasted but driving around in my forest-fire-smoke-filled valley at 95 degrees made it extra unpleasant.
This experience reminded me of when I owned a classic FJ60 Land Cruiser and I belonged to "ih8mud.com"; the after-market "upgrade" ideas on that site, which always turned out to be much more work than described and somehow my situation had some special problem that wasn't mentioned or you had to have a mechanics' knowledge and/or toolset to work around. The older I get, the more I just want stuff to work right on the first try. Moral of the story: stick with OEM.