Could be way off-base here, but I distinctly recall Henning telling me (hell of a nice/helpful guy btw... way more patient with me than I would have been) that the Grams follower requires you to:
1) Pop out the front (older style) or rear (newer style) follower guide rail in the factory mag so the Grams will fit and travel
2) Load long for .40 as the follower guide rail helps keeps standard OAL rounds in-place under pressure.
Somewhat of a neophyte to Tanfos in general, but I recall this as I was all giddy about getting reliable 21 rounds out of factory ammo and placed an order for the innards to stick in my factory mags. Henning called me and steered me away (but correctly) because most of what I fire is factory. No follower guide rail + Grams + factory ammo = sloppy rounds in the mag.
Sound about right to those in the know?