Back on 27's, eh?! If you look at Garvins vintage gallery, here, you should find the correct one:
https://forum.vintageairgunsgallery....na-model-27-2/
Not sure which diopter the 27 was compatible with then, possibly pre-diopter 60, maybe the unit more commonly seen on early 35's. The Diopter 60 should fit the rail.