I had a similar issue to this post by /u/dt_mt2014
I have a 6-pin Schlage key that I wanted to use in a 5-pin SC1 knob. However, the residential knob is too short to accept a 6-pin SC4 KiK cylinder, so a cylinder swap wasn’t possible. Likewise, I didn’t find much online when searching for alternate solutions.
However, it turns out that the blocking item for the longer key is simply the bottom center of the cylinder tailpiece.
So, with some careful filing of the tailpiece, the longer SC4 key can pass under the tailpiece and will protrude slightly beyond the back of the keyway, but it doesn’t protrude enough to extend beyond the plane of the tailcap.
Therefore, pinning the 5-pin KiK cylinder to the first 5 pins of the 6-pin SC4 key allows the lock to operate as normal with both a 5-pin and 6-pin key.
A picture to show the filing work on the tailpiece (from ~4 o’clock to 6 o’clock). ~20 mins with small triangle / half-moon files. It would have been faster if I were less cautious. 🙂
Leaving this here in case it helps someone in the future.