I'm not a helmet expert but my experience is that even on the clear shells, the Steelers decal was always put on top of the shell. However, that doesn't mean that there wasn't a logo underneath as well. I think with the stenciled-font logo that these could have been put on the helmet and then during games a decal with the correct font was placed over it. Those decals then were eventually removed, leaving the logo that you see under the clear shell. Stickers were replaced frequently. This is the one plausible explanation in my opinion as I also have yet to see a photo of a stenciled-font logo in action. Try to see if there's any hint of sticky residue on top of the logo.