I have an SMC Takumar 55mm f2 lens, so I took it apart to find the ring that limits it to f2. Disassembly is typical Asahi/Pentax: remove name ring, filter ring and unscrew front lens group to see this:
The ring is the part just above the aperture. It would have been nice to just pop it out but it doesn't do that. I then removed the rear lens group - no glass to scratch. Then unscrew the three common screws in front and the whole central barrel can slide out the front. Here is the barrel:
On the side you'll see three tiny common grub screws that hold in the evil part. But this part also helps hold the aperture blades in. It's hard to remove without having a few blades fall out too. Reinstalling them is a bit fussy but not impossible. Here's the part:
And with it removed. You can see I got fingerprints all over the blades but they're all in place.
To convert this lens to an f1.8 lens, I can file the opening in the evil part so it is larger. The aperture ring does allow the lens to open to f1.8 when it's set at f2. Therefore, Asahi/Pentax only had to have three parts that were different between f1.8 and f2 lenses: the part with the f2-size hole, an aperture ring labeled f2, and a name ring labeled 1:2/55. The Takumar versions often had the "1:2/55" text in yellow, so a bit of different paint too.
I got this lens as an extra in a package deal and I meant to test it against the 1.8/55. I don't need to do that now, but I haven't decided if filing a bigger hole will help or hurt its resale value.