Denny, wouldn't it be possible to put some registration marks on each page, to force the printer to scale correctly even if "shrink to fit" is set?

There's an issue with A4 vs. Letter size, but I think we can get around even that. Let's see... A4 is 210 × 297 mm, Letter is 216 × 279 mm. So if you set registration marks at 210 x 279 mm, it should scale correctly on either page size.

That sound feasible? or would non-printable margins etc. screw things up anyway?

Definitely not an issue with the PDF itself, I've built a totally workable bone bridge. Not my best build, but that's merely MY fault.

My money's on printing error, but it's also just possible that you're trying to line up parts that don't line up right.