Difference between the Marinair Lo1166 and Lo1166A RW lies in the following:
Lo1166:
Primary impedance: 1.5kΩ
Secondary impedance: 600Ω (balanced)
Transformation ratio: 4:1
Frequency range: 20Hz - 20kHz
Application: Microphone preamplifier, line preamplifier
Common use: Neve 1073, 1084 modules
Lo1166A RW:
Primary impedance: 1.5kΩ
Secondary impedance: 200Ω (balanced)
Transformation ratio: 2.24:1
Frequency range: 20Hz - 20kHz
Application: Compressor, limiter
Common use: Neve 2254, 33609 modules
In short, the Lo1166 has an impedance of 600Ω on the secondary side, making it suitable for microphone and line preamplifiers. The Lo1166A RW has an impedance of 200Ω on the secondary side, which makes it more suitable for compression and limiting.
Additionally, the Lo1166A RW has a slightly wider frequency range than the Lo1166.
Apart from these key differences, both transformers are identical in size, weight and construction.
Note that the turns ratios are
1.58:1 1166
2.74:1 1166A
Transformation Ratio is a term i hgave never heard before and apparently stands for Impedance Ratio or Z ratio, the turns ratio being the sq root of that.