First release the pony truck front bogie by releasing the obvious screw (green arrow indicates)
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I fitted a LENZ silver, note that a LENZ silver direct would not fit.
I always cover my decoders with 13mm heat shrink insulating tubing from Maplin. A quick blast with a hairdryer is sufficient to shrink it around the decoder.
Then I placed the decoder in the smoke box of the 9F...
While I was at it I fitted Kadees in the NEM pockets - they were perhaps very slightly high - see pictures against the height gauge below.
I used number 20s but I think 19s would be fine (I had run out of them so couldn't try)..