Let's have a closer look to the fans.

Starting with the NF-A14, both the FLX and ULN versions retail in similar carton boxes you see below. Each fan is sold online for about $23, which is a reasonable price for 140mm fans.

NF-A14 FLX	NF-A14 ULN

An Ultra-Low-Noise Adaptor (U.L.N.A.) is the extra component you'll get with the NH-A14 FLX version bundle:

1x Noctua NH-A14 FLX fan 1x Low-Noise Adaptor (L.N.A.) 1x Ultra-Low-Noise Adaptor (U.L.N.A.) 1x 3:4-Pin Adaptor 1x 30cm Extension Cable 4x Vibration-Compensators 4x Fan Screws	1x Noctua NH-A14 ULN fan 1x Low-Noise Adaptor (L.N.A.) 1x 3:4 Pin Adaptor 1x 30cm Extension Cable 4x Vibration-Compensators 4x Fan Screws

As we previously mentioned, the main difference between the NF-A14 FLX and ULN versions is their 3-spped and 2-speed fan settings, respectively. The FLX version provides 1200, 1050 and 900 rpm speed settings via the supplied Low-Noise and Ultra-Low-Noise Adaptors, while the ULN version provided 800 and 650 rpm speed settings via the supplied adaptor. Besides that, the fans look similar:

NF-A14 FLX	NF-A14 ULN

The Noctua NF-A14 series has a square 140mm design, enhanced with Noctua special features like "Stepped Inlet Design," which adds turbulence to the influx in order to facilitate the transition from laminar flow to turbulent flow, which reduces tonal intake noise, improves flow attachment and increases suction capacity, especially in space restricted environments.

The fans are sized at 140x140x25 mm and can be installed on rather large cpu coolers like the Noctua NH-D14. They have a square 140mm frame with 140mm mounting holes (124.5mm spacing) makes it a replacement for 140mm case fans and also compatible with 140mm water cooling radiator.

Each corner of the fans' frames includes anti-vibration pads for quieter operation. The fans also feature the second generation of Noctua’s SSO bearing. With SSO2, the rear magnet is placed closer to the axis to provide even better stabilization, precision and durability.