This problem is SOLVED!
Thanks to Dave, Roger and Leonard for your suggestions!
I know you will ask now... What was it?
I can't tell for sure... :-( The problem simply disappeared in the middle of my troubleshooting.
But here's the story (or the puzzle, if you like).
Checked all L.V.P.S. voltages and they checked good... a a couple of tenths of volts down overall. Nothing to cause concern but surely will require calibration later on.
Then, went on to probe the + and - inputs of the Hor. Amplifier board (pins 2 and 15 of U510) and, there was an upside-down signal on pin 2 and 0V on pin 15.
That was a relief, because I was thinking that U510 could be bad... but signals were already bad at its input, and explained all symptoms (compressed sweep and backwards)..
But... I knew the TB plugin was fine, because I did check it on the vertical slot, and it was producing a nice dot going upwards and crossing the whole screen even at x1 setting... So what there could be left?
Only the cable between the Backplane board and the Hor. Amp. board (P51 to R501)!!!
Double-checked the orientation of the P501 header at the Hor. Amp end, and it was inserted correctly. This could only leave the other end P51 (at the backplane).
Checking the other end would be tougher... it's boxed up between the P.S. Transformer and the backplane.
Unscrewed the P.S. cage and pulled it out just enough to clear access to the P51 header and... Surprisingly... P51 was ALSO inserted correctly and it was firmly inserted.
OK. Maybe it's a mistake in the manual... So I checked continuity all the way from the edge connector pins on the backplane, to the pins at P51, and then all the way to the other side of the cable, at P501, and all checked right...
I was heading to dead end: There was no swap of + and - signals, and there was no discontinuity of either signal or their GND shields...! Shoot!
Reinserted back P51 and P501, Pushed back the P.S. cage into place and screwed it again thinking...
Well... let's continue troubleshooting... Maybe the -minus input of the Hor Amp is shorted to ground and maybe this short was causing the signal at the plus input to become inverted (Long shot, I thought...).
Fired the scope back on just to see a nice sweep trace crossing all the screen... Immediately thought that x10 was engaged but... NO... It was at x1!!! And it was not going backwards anymore.
So, Hor. sweep got fixed, but I can't tell how or why.
It surely had something to do with that cable, because it was the only thing that I removed and reseated...But it remains a mistery.