Ok, now I see the confusion. In North America, every Reynolds-based model in 1974 had the 531 decal on the seat tube except the PX. So we're both essentially correct: Your PX would have it on the downtube in 1974, and in '75 they moved the Reynolds sticker to the downtube on all models (except the PX10 which already had it there).

I had to go back and reference on it - the '74 catalog is actually a '74 catalog. Date is clearly printed on cover at the BBP website at least, and you can see the dissimilarity in the 531 decal: Peugeot 1974 USA Brochure