Couv, on my MK10 I immediately mounted a brand new 300-bars Scubapro Universal DIN kit, which here in Italy costs just 37 Eur. Works great !!!
I hoped to avoid the need to make not-original modifications mounting such an universal kit on two of my MK5s, so I ordered a couple of A166, but it was a big error.
I have here around another Scubapro universal kit, albeit it is the older 200-bar version. Here you see it side-by-side with the 300 bar kit, before mounting the latter on the MK10. As you see, it has the proper O-ring after the threads:
I already attempted to mount it on one of my older MK5, but it requires a washer (as you did already point out) for avoiding that the screw goes too much inside the reg body. And even with that, if keeping the brass saddle, the knob gets locked against it, so I should use it without the saddle, which I do not like too much.
I searched a lot on the Internet, and found a couple of interesting photos. The first shows a MK5 with the original DIN kit for it (part no. 10-105-030) and brass knob mounted:
But from this photo it is unclear if the kit is mounted with the saddle, or not. Possibly not!
The second shows the retainer being part of that original SP kit (part no. 10-105-116):
This looks entirely different from the retainer of the Trident A166, shown here:
So possibly the best solution is to buy two new 300 bars Scubapro universal kits, and mount them on my MK5s with the trick of a proper brass washer for impeding to the retainer to thread-in too much.
But I fear that also with the washer the saddle will cause the knob to lock, and to be forced to mount them without saddle, as it happens with the older 200-bar universal kit.
I will report here how it works when they arrive.