I canned butter 1n 1999 that's still very good. I used 12 oz straight side jelly jars. Buying butter on sale is still @ $2 a #, less in couple of case lots.
I sterilize jars by boiling, then upside down in gas oven on 200, butter is melted, temp checked to be 240, skimmed a few times until clear, then poured into jars to 1/2" space, lids come out of simmering water bath, with magnetic lid lifter, then snugged down. This is now Ghee, lasts a LONG time, and is very expensive in the specialty stores. I've used both salted and unsalted butter, don't see any difference.
For this, you don't need a pressure canner, nor for jams, jellies, pickles.
+ 1 and repeat: Old recipe times and temps not safe, I recall a vichiossee (sp) soup processor who discovered a resistant botulism strain. I will not touch shellfish for this reason.