Any P51 experts out here?

I'm researching something and can use some assistance if anyone has the
expertise or the inclination to be of assistance with this.
Apparently there was a training manual put out during the forties on the
P51 Mustang (not the airplane's dash 1 which totally contridicts this
manual) that said the 51 could NOT hold or maintain a slip.
I'm interested in any information on that manual, and/or the reasons for
this statement.
I already know the Mustang can be slipped as I've slipped it many times.
What I need is origin information on this exact training manual and any
reasoning for the no slip ability statement being in that manual.
Thank you
Dudley Henriques
International Fighter Pilots Fellowship
Commercial Pilot/CFI Retired
for private email; make necessary changes between ( )
dhenriques(at)(delete all this)earthlink(dot)net