Finding 60 year old appliance parts is one of the many things AW.org is all about.
Perhaps someone that can help will see the thread, finally after almost 2 years, don’t give up hope.
But you very well may have to improvise, or just leave the butter dish out for a while before you need the butter to be spreadable. Thats what I do.
You can also quickly soften butter to a spreadable consistency by placing a stick in the MW oven at 10% power for 1 min. If its still not soft enough give it another 20-30 seconds at 10% power. I do this all the time for baking and it works like a charm.
HTH
Eddie
Eddie