While I was in SF, I found some refills for Uni-Ball Signo. I couldn’t remember if these would fit in my Render K but I was willing to take a chance. It was blue-black ink in my favorite 0.38 size. At less than $2, it was a gamble I was willing to take.
The package was labelled UM-151 0.38mm. Jet Pens does not seem to stock this particular flavor refill but Maido’s online shop, MyMaido does stock them.
UPDATE: Eagle-eyed reader, Adamfmoore found the proper refill on JetPens. It’s the Uni-ball Signo UMR-1 Refill and it sells for $1.65.
I discovered that the refill was about an 1/8″ too long so I trimmed it down with a trusty X-acto blade. The plastic is quite hard so if you try this yourself, be careful! It would be easy to slice your finger in attempting this. Once trimmed, the Render K screwed closed easily and voila! Deep green pen with blue-black silky ink. JOY!