>Since I do not know a US close equivalent...

one-shot

>The logic of its construction...

it's from a little used form of the adverb off:
with a preceding numeral to represent a quantity in production or manufacture, I'd look at it as 'off the (normal) production stream/line'.
1947 Crowther & Whiddington Science at War Manufacturers found it very difficult to give up mass production, in order to make the 200 or so sets ‘off’. 1970 Cabinet Maker & Retail Furnisher Without barrier coats mould breakdown can start after 60 units off. 1973 Physics Bull., Kienzle printers. 6 off, surplus to manufacturing requirements.
one-off therefore being the most extreme level of 'off' you can attain. :)
None of the motor-cars illustrated is a standardized mass-produced model; all are expensive, specialized, handicraft one-offs which can justly be compared to the Parthenon because, like it, they are unique works of handmade art.