B-girll, cool old stuff is always fun, and if the portacatena exists and you have one, why not try to use it and just experience what the idea was and how it works. We're not really just into great old bikes for modern utility, some of it is just to feel what was good back in the day. Teh only way to experience the greatness, such as it may be, of old stuff is to build it up and ride it.
My Mondonico has a portacatena hole too, and I feel like it's incomplete if there isn't a gadget installed in that hole. Antonio Mondonico drilled it and threaded if for some good reason, right? Even if my riding does not require one. It also does not require a formerly state-of-the-art Italian stage racer.
In the days when 5-blocks were finally waning, my bike was built with 126. It came to me with a Shimano 7-speed installed and always worked well with 7's. Apparently it should have also worked well with a 5+Porta or a 6+Porta.
Have at it!