Spanish bakery Bimbo produced three Letraset promotions in the form of booklets they called "Bimborama". Originally they all appeared here on a single page, but recently so much additional material has shown up that I've had to split the article into three pages — one for each album:

"The cowboy who draws faster than his shadow."

Back page ad. Okay… so the bad news is that here are some Letraset transfers I have no other record of; one transfer with every packet (or double packet) of Bony, Tigreton, or Bucaneros. The good news is: yay, Letraset transfers!
However, it's as likely as not that the transfers shown here are for demonstration purposes, & weren't genuine transfers actually available; in other words, this could just be an advertising company mock-up to show you how to get hold of the transfers to rub down in your Bimborama albums.

The Bimborama is divided into five backgrounds for the transfers, labelled "A" "B" "C" "D" & "E". Each transfer has one of these letters printed on it, so you won't get it too horribly wrong.
I don't have the Bimborama itself, but I have managed to obtain a full set of twenty-four transfers with their background page-identifying letters clearly printed at the bottom right corner of each sheet:

Unfortunately, after receiving them it became clear that 'complete' was an exaggeration. No apology from the seller!
But here are the missing transfers, as sent in by our contributors. Note that the one in the middle is the same transfer being rubbed down in the TV advert from the Milky Bar promotion shown on the previous page!

…That's twenty-seven sheets, so there may have been a twenty-eighth; if so, I've seen no sign of it. If there were multiple sheets & some transfers appeared more frequently than others, twenty-seven could well have been the full complement after all.
As it stands, we have:
The Lucky Luke Bimborama was also reprinted as part of the last "How Things Work" album (No.3). I'm not sure how Lucky Luke is supposed to demonstrate things & their workings, but there you go…


Although the backgrounds are no longer in colour, as a bonus you can see the transfers applied by the owner!
Picture Credit: The SPLAT Scan Archives — several anonymous Spanish Friends of SPLAT!