New to me rusty grandpa bear

The right side was replaced with a newer cap. Barr Castings in Portland Oregon has the original molds and offers both 4 and 5 fin in the material of your choice. Specify that you want the draft caps WITH steel insert to prevent thread wear. (and keep them greased) They now have an online store.
They sell them cheaper cast brand new than what I've seen them sell for on eBay! So you should be able to sell your old ones for more than a matching set will cost you!
http://barr-casting-inc-store.myshopify.com/collections/all?page=1

They can make them in what ever material you want. Probably have aluminum in stock since they are the cheapest and most common.

Fabricators used up the old stock of what they had for handles, springs and dampers before buying the new styles. The handles were straight at first with stainless springs tightly wound. They had pipe caps for dampers until the 4 fin was patented by Bob's dad Nov. '76. Later, the handles were bent like yours with springs facing forward and springs were plated with nickel or brass like yours. So by the time yours was built, (bent handles with newer springs) the 5 fin caps existed, but this fabricator could have been using the old 4 fin before switching to the new 5 fin. So it was possible to have had either. Most by 1978 had the newer configuration of handle, spring and cap that stayed that way until closing in 1988. So it just looks "right" with 5 fins with your later springs and handles, and 4 fins with the older straight handle and older springs.



First style trees for 1976 and early '77 did not match Grandma style smaller trees. Some fabricators were using up solid ball handles and making their own pipe cap dampers.

Old configuration with smaller trees.
The reason for fins was cooling and being able to turn with your foot. I personally think the 4 fin is easier to use with your foot than 5 fin.