I expect LFB may perhaps have been prepared to make exceptions if there was a significant cost benefit. They apparently did this when they bought the old Dennis Camiva TTLs - which were originally specified to be the more acceptable Magirus ladders. And again, when they bought so many Shelvoke "look-alikes" instead of the more expensive Dennis alternatives. These met the paper specifications but were reportedly very inferior in practice. After all, reducing tank capacity by 450 litres only involves shortening the overflow pipe a little bit, and other necessary mods might have been within the scope of the brigade workshops...
Added by Rob Johnson on 14 June 2022.