There were two different rear sights used on the Model 46, which was made from 1959-1968. One style for the 5" and 7" barrels and a different style for the 5.5" heavy barrel. The good news is they are the same sights used on the Model 41 barrels of the same vintage so you should be able to find a used or new old stock one by checking with one of the gun parts companies such as Numrich or trying the online auction sites like eBay.

Here are some Model 41 barrels so you can see the different rear sights. The rear sight on a 5" or 7" barrel would be like the top two barrels and the rear sight on the 5.5" barrel would be like the bottom two. I should also mention that the rear sight used on the 5.5" was also used on the Model 52 so that could increase your search.



As for the holes, they aren't factory and it would be tough for someone to guess which mount would work with them without some pictures and measurements to go off of.