Something they offered early on, but not anymore!
I've had one for quite a few years that is essentially all brass or bronze, except the crankshaft and base, but that is just what they still offer today #3BI, and yes they do offer a #3A in aluminum.
Geoff (ozsteamdemon) had one of the large #1 Steam Drilling Engines that was all bronze and brass if I recall correctly, and that was something to behold. Likely from the same era as your #3.
So .... to answer your question, I'm not sure it could truly be called "Rare" (though it may well be), so much as it is just the early version of the kit. I think we'd all agree that is a great find you've made there ... Congrats!!!