Hi Am,

Stan's correct. No sx-70 back for rb67- only the polaroid backs intended for the Polaroid 100- style pack film. which you mentioned you have!
unfortunately you get two dark bars either side of the image with the fp100!

I should also note that there are two styles of the PX-70 from the impossible project- The 'First Flush'- first emulsion and the 'Push'- being the newer more stable emulsion!

There is also the PX-680 which (from what I've seen online) has more vibrant saturated colors- but will need an ND4 filter on a SX-70 to be used. or a 600 polaroid camera!