The sternpost gudgeon (at the bottom of the photo) is unique. It is just the ticket for down low on a wherry transom where it's narrow or on a sternpost, provided the face is at least 3/4" wide. If you think they're unusual, you're right. It is our design, and we make them. Like our other rudder fittings it is bored for use with 5/16" rod. In the photo below, the sternpost gudgeon is the lower one fastened to the transom.
This photo of a Duck Trap Wherry transom shows transom gudgeons above and below. Had this been a LincolnvilleSalmon Wherry with her much narrower transom, she would have to have a sternpost gudgeon for the lower fitting because there simply isn't room enough for a horizontal (transom) gudgeon.
Yes, you can use these on the curved sternpost of a Double Ender (a.k.a. Peapod for those of you who harken from outside of Maine), but you will have to enlarge the holes and bend the 5/16" connecting rod so that your rudder will be able to jump when it strikes, and then drop right back down into position afterwards. It wants to be a loose fit to function properly. Alternately, you could flatten a short section of the sternpost profile so that the sternpost gudgeons can be installed on the same axis.
The price shown is per piece. If you prefer to do your own drilling, for whatever reason,you can order the unfinished castings.