[AMRadio] Unusual transformer

[Robert Nickels]

I'm puzzling over a transformer I found in my junk, which is clearly 
identified as a Thoardson T-21M66, but even after finding it listed in 
the 1947 catalog online, I'm still not sure how it was used.     It is 
described as a "Matching Line to RF Load Modulation (M) Transformer" and 
has a single 500 ohm primary (that would be the "line" part), and five 
secondary taps that are only described as "5M-6M-7M-8M-9M-10M" which 
must be the "RF Matching" part, but darned if I know what these values 
are or how the transformer would be used.   The audio power rating is 50 
watts and the secondary is good for 200MA.    A scan of the actual 
catalog section can be seen here:

https://i.imgur.com/2eyXns2.png

I've included the multi-match modulation transformer section above so 
you can see what the column headings refer to.   Can anyone shed light 
on what this transformer was used for and what the secondary values mean?

Thanks and 73,

Bob W9RAN