Finally, I may have something new to contribute!
One of the clearest tells for an RXF comes from the serial numbers. So far, I have seen:
3x F77S7878
3x D55P5858
2x E66R6868
1x A11L1818
Letters occupy the first and fourth places. So far they have been 11 to 13 letters apart in the alphabet.
With the numbers, only one is variable and this occupies the second, third, fifth, and seventh places. So far, it looks like the variable number equates to the first letter + 1 ie. F is the 6th letter of the alphabet so the variable number is 6+1=7. The exception so far is the A which retains the variable number as 1.
The sixth and eight places are always an 8.
The numbers appear random enough at first glance but this pattern should help us spot RFX reps in comparison shots in the future.