Pretty sure that only a 10 leaf spring was used on the Model A Ford CAR.
Sometime around 1929 a Service Bulletin announcement indicated that "12 Leaf Spring is now made available to those cars subject to severe service."
Meanwhile, the AA truck soldiered along from the beginning with a front spring that was the same width as the rear spring of the Model A Car. (2-1/4")
I have seen the Eaton "selector" and while the 12 leaf spring is shown, it has a "spring rate" of 343 lbs/inch. The 10 leaf spring has a spring rate of 315 lbs/inch. So it appears the 12 leaf spring is "stiffer."
https://eatondetroitspring.com/db/es-partsinquiry2.php?syear=1929&smake=Ford&smodel=Model%20A&sbody=Tudor+Sedan&seng=L4+3.3L+-+200.5cid
This may or may not affect how you perceive your ride - your car will just ride at a different "height."
Curiously, Eaton Spring on their site indicates springs are made to original blueprints. This statement is at odds with a respondent here who indicated that Eaton had said to him they couldn't be troubled to produce the Model A spring in varying leaf thicknesses (as Travis Cook has) due to the cost, but that their "engineered" springs embodied the newest (and ostensibly better) technology - and allowed them adjustment to "ride height."
My 1970s era bought "JC Whitney" replacement spring "emulates" the Model A spring to include slightly tapered leaves. All 10 leaves are the same thickness. I have compared it "deflection wise" with an original but "worn" Model A spring. The JCWhitney is considerably "stiffer" but some of this may come from a now "relaxed" older Model A example.
As I've said elsewhere - JC Whitney likely a trailer spring of similar size.
And spring tensioning is not forever.
Joe K