can this mount 140mm fans to a 120mm radiator?

It cannot. The poles would raise the 140mm fan off of the radiator, killing static pressure and causing clearance issues

Does a 120x15mm Noctua fan fit?

The NA-SFMA1 has been specifically designed to ensure an optimal fit of the NF-A12x25 fan on watercooling radiators. It is also compatible with the NF-F12, NF-S12A and NF-P12 redux models as well as some 120mm fans from other manufacturers, but due to the special adaption to the NF-A12x25's frame shape, not all 120mm fans fit.

Questions & Answers: Noctua NA-SFMA1, 140 to 120mm Fan Mounting Adaptors for Watercooling Radiators (2 pieces, Black)

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Noctua NA-SFMA1, 140 to 120mm Fan Mounting Adaptors for Watercooling Radiators (2 pieces, Black)

Allows the installation of Noctua NF-A12x25 120mm fans on watercooling radiators that are designed for 140mm fans
Improves the performance of all-in-one (AIO) or custom (DIY) 140mm-based watercooling systems through the superior efficiency of the NF-A12x25 fan!
Compatible with the NF-A12x25, NF-F12, NF-S12A and NF-P12 redux fans
Includes two NA-FMA1 adaptors and eight NA-AV4 anti-vibrations mounts for two fans
Trusted Noctua quality backed up by 6-year manufacturer’s warranty

$12^.95 –