I'm not 100% sure of the MF-14 being able to imprint in-between the frames. I didn't pursue an MF-14 back primarily because of that reason, instead, I managed to get the MF-18 data back which does have the capacity to imprint data in-between the frames.
I will mention that I cannot remember the single time I saw and handled an MF-14 data back, whether or not this is completely correct. But I do seem to remember the imprint window on the data back was in a landscape format.
To that end, I have just checked my Nikon Compendium:- ISBN 1-897802-02-1, page 97, which confirms the MF-18 is the data back that imprints in-between the frames, compared to the MF-14 of which there is no mention of it's ability to imprint in-between frames.