Old Pentax manual-focus bayonet lenses are divided into K, M, and A types. The K and M only have mechanical aperture coupling, so on the "crippled KAF2" or "crippled KAF3" mounts, which are the mounts of modern Pentax SLR's and DSLR's, K and M lenses won't meter except in stop-down mode.
"A" lenses have a pattern of contacts on their bayonet which communicate their maximum aperture, which allows full program operation, P-TTL flash mode, and so on (basically everything except autofocus).
There's a mod floating around which supposedly tricks the camera into thinking any lens mounted on it is an "A" lens. This is easy to do because "A" lenses have a recessed pin which pops out when the aperture ring is moved to the "A" position. The moving pin is called the "A" pin. So to dupe the Pentax DSLR into thinking any lens mounted on it, is an "A" lens, all we have to do is short the "A" pin on the camera mount.
Here's your typical crippled KAF3 mount:
The "A" pin is the semi-recessed one that is third in line from the left along the rank of pins next to the lens release button. It's distinctive because it's flat and recessed, unlike the rest of the pins which stick out and are round. We need to short this pin in order to "tell" the camera that the lens attached is an A lens.
After getting the metal bayonet off. Note that there's a very thin metal ring which must not be molested or removed; there is a thin wire soldered to this ring and that wire had better not get cut!
This is the metal bayonet, stuff a piece of aluminum foil into the "A" pin hole.
Another approach is to put a tiny aluminum foil "cap" on top of the "A" pin itself:
Put the metal bayonet back (don't force it if it doesn't go in nicely, the metal washer underneath must fit neatly in for the bayonet mount to go down nicely). Torque in the screws as if it were a tire, opposite screws at a time, finger-tight only.
This is what the mod looks like:
An ancient K-mount Vivitar 90mm macro:
And as we can see, the camera thinks it's an "A" lens!
Now for a few caveats: fixing the lens at "A" doesn't really do anything. P-TTL metering results in massive over-exposure. The camera thinks that the lens is an f/1.2 lens (the fastest K-mount built) because your SMC-K or SMC-M doesn't have the aperture-setting pins, so program modes also result in massive metering errors.
Green button no longer works because the camera doesn't try to stop-down the lens anymore (it's an "A" lens, right?)
So the only benefit I can think of, is that aperture data is now recorded in the EXIF. But you have to make sure that your aperture set on the camera, matches the aperture ring on the lens. Also, since "A" lenses have a differently-designed diaphragm from K- and M-lenses, there will be exposure errors.
Overall, not a worthwhile mod but heck, it takes ten minutes.
Update: after a bit more testing, it seems that P-TTL flash does work, but only with wider lenses. I'll try to see if this mod is actually worth keeping, before I dismantle it.