In years of experience with working using his springs, the ones he makes in the same batch (he typically makes a lot of them at a time from what he's written other places) are so-close in size that he could easily make all the guides the exact same size and the kits would still come out just right. In other words, you may call that a 'generic fit', but I trust them enough to call them a custom fit. Going further, the springs made in successive batches are usually so close in size to previous batches of the same type of spring that you probably wouldn't notice a difference between them, either. Besides, varying the guide size by a couple of thou. doesn't really matter much when it comes to performance in the real world.
If he corrects me in any way, I apologize in advance.
Since you asked, JM's answers are usually brief and to the point, but you'll get an answer. Don't push if he doesn't elaborate--he gets a ton of questions.
Safe and Happy Shooting!
Ed Krzynowek, The Airgun Tune-Meister
"We can rebuild the squirrel. Make him stronger, faster...We have the technology"---Skyler M.