got a band-on Campag shifter and very happy to swap for a braze-on.... because the new bike does have the bosses and adding a band-on makes it look very odd!
Not sure what year you have but in the 70's all the braze on did was mimic the boss on the band and they were completely interchangable..... unscrew bits from band and plonk onto the boss on the frame
Any pics?
Sometimes the plate with the hole for the cable became jammed so looked like it was part of the band. Or more commonly the plate jammed onto the braze on and got re-painted come respray time LOL
You may find that all you need is 1x pair 600/2 & 611/2 backplates/stops like the square-centred ones at the bottom of this pic from Velobase:
Nigel at Campyoldy sells them for a whopping £2.40 a pair.
You're right in thinking that the equivalent part on the band of some earlier shifter pairs is non-detachable. In some cases, the thicker washers/collars shown above the backplates can also be integral to the band assembly. If that's the case with yours then Nigel also has the parts you need: #661 Brazed Boss Collar at £4.50/pr. He only has the more recent plastic ones and not steel as pictured, but once assembled you'll never know.
I know all this as I've recently covered the same questions with Nigel while trying to complete a set of braze-on gear levers!
Also, to be period correct you should have lever blades with the raised logos as pictured here and not the stamped ones from a later period. If you have the later stamped lever blades, then I would consider swapping the earlier pair that I have, if you're interested.