That's not a common gun...5 inch N Frame. The k frame swims around in it.
Definitely for the 5" RM. (If you don't have one handy, now you have a legitimate, defensible need to buy one.)Probably for a HD with the common 5 inch barrel. Or possibly a RM with a 5 inch.
And a fine book it is. A necessity for a serious collector, and a very nice, beautiful coffee table book for a dilettante like myself..... This kind of info is in the book Holstory by turnerriver and me.