Update... tick tock tick tock 
This story is going to make me suffer  until the very end! :-)

The technician who was supposed to assemble the cable  is a nice  old  technician who has worked for 50 years on  radio and TV repairs and he said to me, "Oh Mario, I'm sorry, but  my hands and my eyesight  no longer allow me to do this kind of work. The  8 pins  to be soldered are too small, you need a steady hand.  Let's wait for the young guy who is just learning  the job to come back,  he's  away for a few days."
And I wait...

Maybe this has gone on long enough…

I've never had the pleasure of traveling to Italy, BUT I have a feeling that the country is advanced beyond pasta and wine in many ways - including computer equipment.

I can find this damn cable on ebay in the US in 10 seconds flat.
I have a feeling that you can easily buy these on ebay in Italy… or at least you should be able to if you can simply do a proper search for it.
If I could read and write Italian I would try to search on ebay.it myself but oh beh, peccato… I can't.

I searched in the US for a "Macintosh mini-DIN crossover serial cable". Translate that to Italian and you may just find what you need. In fact:
You can also simply search for a "Macintosh Imagewriter" OR "Macintosh printer" mini-DIN cable because those are also crossover cables.

I'm just sayin' here that considering the complexity of what you're attempting to put together, this effing little cable should NOT be the hardest part.

Here's just one of the US ebay finds: https://www.ebay.com/itm/232343545580?

buona fortuna a te

Hack & Whack / Temp "Test Approach"

If one has a straight-through cable to butcher.

Why not cut the cable and simply twist test necessary pairs together?
Realize this isn’t a permanent solution — perhaps a good temporary test?

 

    Black to Brown / Yellow to Red / Purple to Green / Blue to Blue / Orange to Orange

---

Color coded wires from a straight-through cable, with associated pinout(s):



Cut the cable close enough to one end that it could then afterwards,
possibly be soldered to a new plug. *Of course, before cutting it “close
enough” / also allow an ample length for this possible “twisted pair test”.

It might look like absolute hell with all the wires exposed & twisted together
in this manner, until a more “finished” cable could be created or acquired?

---

OR Does an old Apple printer cable 590-0552-A (below)
equal an Apple Mini DIN 8 Crossover Cable TX-RX?

Looks like this one does. (Or so says side-by-side testing with a multimeter.)
Had a little free time last night in the now, seldom visited, cable cavern here.


*Above not color coded to match the actual wiring colors.

Had one of these in the “boneyard” and continuity testing provided the above results
… which seems to “fit” the descriptions of the now seemingly rare crossover cable.

Another different cable here, marked F2V024-06, w/ a taped label of “IMG-II” attached;
its continuity tested out the same as the above shown 590-0552-A cable.

But if you really want to get out in the proverbial weeds with this…
our distinguished forum member part12studios (PM him) chased
this very same problem waay back in the year 2016.

Here:
https://www.doityourself.com/forum/computers-internet-capable-devices-peripherals/566685-8-pin-null-round-modem-cable-wiring-help-needed.html

But seriously now -- and yet again... ALL kudos to GaryN!

Well it looks like another waste of time on a down-the-rabbit-hole theory, concerning any motherboard creation date similarities having any real bearing on the MDD J20 RAM slot problem. Mardeec’s mobo info on top and mine directly underneath it. Both with the J20 slot problem.



So it seems entirely more likely, as we had previously suspected (me, peeperpc and GaryN) that the J20 slot suffers (over time und zee heat) from its close proximity to the GPU. Whether that GPU is some high performance (and thus, likely operationally hotter GPU) or some perhaps lesser capable (overall cooler running) GPU.

Again, please refer to Gary’s extremely well-explained post on the subject here:
https://macos9lives.com/smforum/index.php?topic=7637.msg59356#msg59356

Don’t we always have f-f-f-fun?

Ahh, to only “suffer the slings and arrows of outrageous fortune”!

Amen fratello!

  

When the computers were being actively used and supported, the null-modem MiniDIN-8 was the only type of serial cable a user was likely to find in a computer store. Far from being some kind of exotic thing they had to solder themselves.

definetly a blue ice. no idea about their exact models and software compatibility.

They look almost identical but with big granddaughterboard

Possibly "related"?



See / read: https://www.provideocoalition.com/after-effects-performance-part-4-bottlenecks-and-busses/

I ordered G4 Sawtooth and i can't identify this expansion card that i will get with it

Update... tick tock tick tock 
This story is going to make me suffer  until the very end! :-)

The technician who was supposed to assemble the cable  is a nice  old  technician who has worked for 50 years on  radio and TV repairs and he said to me, "Oh Mario, I'm sorry, but  my hands and my eyesight  no longer allow me to do this kind of work. The  8 pins  to be soldered are too small, you need a steady hand.  Let's wait for the young guy who is just learning  the job to come back,  he's  away for a few days."
And I wait...