Originally posted by Canada_Rockies You need a 1.4x-S converter. The 1.4x-L will not fit inside the throat of the lens. My M400 has the same problem, and I went through the hassle and expense of getting a 1.4x-S first.
This isn't quite right, at least according to the converter manual. The 1.4x-L, as you correctly state, does not fit on the M400, however it does fit on the A400. At least the manual says so. See the bottom left of page two in the manual, link below.
http://www.pentax-manuals.com/manuals/accessories/converter_a_s.pdf
I went through the same thing as you. Had a M400, acquired a pristine 1.4x-L from KEH . . . and it wouldn't fit. Sent it back. I've since sold the M400 to a collector and just this year, snagged a pretty good copy of the A400. Bummer! 1.4x-L lenses are hard to find now and really expensive. (I have the other three.)