Phil, the Ilford site lists an anti-halation backing for roll (120) and sheet film but none for 35mm. Makes sense as presoak of 35mm pours out clear while presoak of sheet and roll pours out black. I use all three types of FP4.

The acetate they use on 35mm has some kind of tinting that isn't on the roll or sheet film but to me it looks neutral grey, not blue. Densitometer readings on roll and sheet film (FB+Fog) are half that of the tinted 35mm acetate. Readings of .16+/- for roll and sheet versus .32+/- on my old analoge weston densitometer.

Don't ask me why they do this or what affect it has, but if anybody knows please chime in.