You guy's like that wiring job to eh? My '91 Prestina came the same wiring job. . . That must be a stock factory wiring job. I did not have the retention knot though.
Several of the bolts on my Prestina were firmly rusted into place and no amount of penetrating oil would do the job. I applied it several different brands multiple times a day and a few just did not want to budge. I eventually found that using an incredibly tight pair of locking vice pliers worked much better than a wrench on the stuck ones. After gripping them with the vice pliers I would then hit the pliers with a rubber mallet. I did this for a week or so, got a few out and then moved on to a more aggressive method. I ended up getting the other bolts off using 2 cans of bolt freeze from my local automotive supply warehouse. It gets the rust too such a low temperature that it creates pores in it, allowing you to apply more oil and hopefully remove them. I could not remove one of the bolts, so I just ended up drilling it out and rethreading the hole.
In regard to the piston. No, you are not missing a gasket. I asked Scott, the head tech at Allann Bros, the same question when I was rebuilding mine and he said that they were designed this way and that every one he had ever worked on had this setup. I believe he stated a reason for it at the time, but I cannot recall the specifics.
Mine actually came with a Sirai. The individuals I purchased my machine from were the original owners and had never replaced anything on it. Heck with the condition it was in when I got it I would be shocked if they had ever used it. It looked like it was sitting on (not in) a barn for 10 years. The Sirai was shockingly clean as compared to everything else on the inside and outside of the machine. With that said, it is a '91, so Conti may have switched parts somewhere along the line.
I do know Cara was actually an espresso machine distributor out of San Francisco. I noticed the company name on he front of your machine. They purchased and rebadged machines from Conti. The '52 2-group Empress I have is a La Cara. Shortly after purchasing it I sought out 'La Cara' to get more details on my machine. I ended up getting in touch with the son on his personal cell phone, he was quite shocked that I had tracked him down. He said that his father had started the company and they no longer carried Conti's. He did not have any detailed sales records to refer back to either.
How did the parts search go? Were you able to source all original parts? If so, I would encourage you to keep the machine in it's original condition and forgo the fancy modifications. There are so few Prestinas floating around it would be a shame to mod it from it's original design. With that said it is your machine, so if you are going to mod it then go big with it. It is like modifying a Ferrari. If you are going to do it you have to go real big!
P.S. - Great video on the Prestina Doug & Barb! I remember the tight setup of the front face plate too well. It made it quite fun to disassemble / reassemble. I really enjoyed the overview of the group. Nice Job! I tried leaving the comment on youtube, but it would not post for some reason