With your Technics 1500:
1. set it for small reels, 2 track playback, and the appropriate speed.
2. wind the "backwards playing" tape fully onto the take up reel.
3. swap reels and re-thread the tape.
4. the 2 track mono (1/2 track mono = same) tape should now play perfectly forward on the left channel.
5. if the tape was recorded on both sides in mono on your Uher, as others have stated, you would hear "side 1" forward on the left channel and "side 2" backward on the right.
6. IF the Uher was a stereo 1/4 track machine and if you switched the Technics to the 1/4 track (4 track = same) playback head you would only get forward sound. It still might be only on one side of the tape or the other and only on one channel or the other depending on which channel you plugged the mic into, if recorded in 4 track mono and not all tracks fully utilized, but not backward. I think this might be the case with your one tape that plays forward on the right channel.
The reason for this mess (as shown by the track and head configuration chart elswhere) is that 4 track stereo (or mono) tapes are interlaced, which means that the "top half" of the tape contains side one left channel and side two right channel, and the "bottom half" (until you turn it over) contains the right channel for side one and the left channel for side two.
One more point (for R2R newbies) "turning the tape over" means flipping the reels and running the tape the opposite way (just as with a cassette). You NEVER turn the tape inside out so it comes off the wrong side of the reel and/or the other side of the tape (shiny on older non-backcoated tapes) faces the heads!
For all here who may care to think it through:
On a 1/4" wide tape the only way to hear backwards sound is to play a 2 track mono tape recorded on both sides on either a 2 track or 4 track stereo machine. The right channel will be backwards material from "side 2".
A 2 track stereo tape will be backwards if played from the wrong end on a 2 track or 4 track stereo machine.
A 2 track mono machine will give you left channel only of a 2 track stereo tape.
A 4 track tape if played on a 2 track mono machine will give left channel forward sound from side one combined with right channel backwards sound from side 2 of the tape. If it is a 2 track stereo machine it will do the above on left and then side 1 right channel forward combined with side 2 left channel backward on the right. If you turn the tape over it just reverses which combination is on which channel.
The only possible scenario for backwards sound on a 4 track tape on a 4 track machine is if you 1. play a 4 channel one direction tape like a quadraphonic tape or multi track tape from a project studio machine like a Teac 3340 from the wrong end in which case all 4 tracks would be backwards (Like a 2 track stereo tape on a 2 track stereo machine from the wrong end). or 2. if you play a double sided 1/4 track stereo tape on such a 4 channel machine, typically "rear" left and right will be the other side of the tape backwards.
OK, I guess there is one more case. Should you ever find a full track mono (whole tape width) tape and play it from the wrong end it would be backwards on anything!