I replaced mine a few years ago. As far as I am aware, there is only one variety for a '51
(there are others for '49' and '50's). I found the replacement on eBay. I think I kept the old one (the mounting "nubs" wear out because it's plastic). Next time I go to my shop, I'll see if I can find it and take a picture.
This?
https://www.ebay.com/itm/115519215421
or maybe this :
https://www.ebay.com/p/1876734639