I need help

The below link goes to a photo of the unit with the metal U plugs installed.

Installed

Dill, the jumpers shown in the photo are not necessary because, as you can see right above the jumpers, the connection  pre-main is made with a switch.

Please, check if you can access the link below. It’s a picture of the rear panel of my amp with the original shorting plugs connected to the Phono 2 input.

https://www.dropbox.com/scl/fi/y3b5ow88y0yn1sl4pdvqw/Capture.JPG?rlkey=tfv2sa95w7iye3mmp9480ii2l&st=waz66buw&dl=0

With the picture and the manual in hand, everything is clear now.

In page 2 of the manual, those are the plastic covers.

IMPORTANT PRECAUTIONS

3. Nine pin jack covers are supplied with the set. Be sure to insert these cover into the unused pin jacks on the rear panel to avoid dust accumulation.

And in page 12, those " jumpers connected to the right and left entry RCA connectors of Phono 2 " are indeed the shorting caps (or shorting plugs) that @dekay mentioned in his earlier post. 

SHORT-CIRCUIT PIN PLUGS
When you unpack the set, you’ll find special pin plugs inserted into the PHONO-2 jacks. Their function is to short-circuit the plus and minus terminals of the input circuit and thus prevent any noise from being generated by them when these jacks are not in use. Therefore, if you use the PHONO-2 jacks, re-position these plugs into the PHONO-1 jacks. Or, when both PHONO-1 and PHONO-2 jacks are used, keep them elsewhere for later use. Do NOT insert them into the output jacks (such as TAPE REC), or the sound from the set will be cut off.

https://steampoweredradio.com/pdf/sansui/manuals/au7700%20operating%20instructions.pdf

Thank you all very much for the opinions and instructions supplied for my questions.

I now feel secure to either, work the amp without the shorting plugs, or installing one with 0Ω.

Best regards

Fabio