I seem to remember using this sight on a Model 66 for a while until one of the older types was found to replace it.

There are a lot more of the older type Original/Diana diopter sights around as they were used on all the Original target rifles (except the Model 75) and were also sold as optional extras and were sometimes suppled with the Model 35 and Model 50 rifles.

This sight was only made for the Model 75 and so there are a lot less of them around.