The 125B may predate the 1590N1. Hammond added a bunch of enclosures in the last decade or so that weren't available when I started building pedals and the N1/BB2/B2/etc... that don't follow the 1590(letter)(letter) format in their names are all relatively recent. Or they may be ported over to the 1590 series from one of Hammond's other lines that was not common in music industry products. Around the same time is when they started offering a few enclosures in half a dozen colors other than black and natural. You're probably right that it was only available with ribs.