The one on the left.
Pilot Protex
You’ll want to search for both protex and protec(s) if you’re looking to buy.
Ok fellas. One of you needs to add an avatar image. These green H’s are confusing me. 
Fixed 
Why is the “Protecs” such a big deal?
I -personally- hate twist-to-extend M.P.s…
Edit: that is why I’ve stayed away from early Uchidas…
Hello??? Is there something I’m missing?
I’ve seen the “Murex”***(im pretty sure that is what it has been called). Compared to the Protecs. Does anyone care to enlighten me why the “PRO” is so beyond the “MR”???
The Protecs is more like a Platinum Z. You twist the halves to first extend the tip AND the knock. Then you click on the knock to advance the lead.
I don’t mind twist to deploy the tip, as long as that’s it. There are some models where you twist to extend the lead with a spring release mechanism (a ratcheting effect). I don’t really care for those as they’re clumsy to do one handed.
The Protecs is twist to deploy. Lead advancement is a standard click. The early Uchida Drawing Sharp models are twist to advance lead.
Note that your typical double-knock pencils are a long push to deploy and short click to advance lead. And by that token, when you do another long push to retract, you inadvertently extend lead. This can sometimes cause lead to poke out of the opening when retracted. So you need to take an added step of tucking the lead back in before releasing to retract. With the Protecs, the retracting is a reverse twist, and so there’s no extra lead extension.
The MR is a nice executive pencil. My favorite is the dotted version.
