I agree, at at least in part. It of course all depends on what sort of jeans you want to make. Conners sewing factory makes 1940's reproductions and they're doing a really smashing job at it too, but if for arguments' sake you would like to make something with a more of a 1920's or earlier look, a single needle like a Singer 31 would be quite appropriate and in fact, combine that with a Singer industrial buttonhole attachment (the granddaddy of the YS Star 4455) and that's about all you need. That said, this would be perfectly feasible as the denims used back then were 11.5 oz max.
But you are completely mistaken on the class 15 and the class 31 being related. The class 15 and the class 16 are related, the 31 was a unique development and while the general form looks similar, there are significant differences with the class 31 being a thoroughly more modern machine. The biggest innovation that was incorporated in the class 31 design was the direct tension unit release, an idea Singer frankly "borrowed" from the design bureau of Wheeler and Wilson, and they would buy the out some 7 years later altogether. The biggest advantage the Singer 31 has over really all it's competition, is the fact that it can handle a 160 needle and 8 cord in the needle and 12 weight in the bobbin without any trouble making it really capable to sew absurdly heavy gear. More than enough for any whimsy 14 oz denim...
I rather think Juki oriented themselves at both the 96 (oh look, more Wheeler and Wilson genius....) and 241 class for their "own"
designs. But that said, before that, Juki copied their hearts out on every Singer design they could get their hands on. So did just about everybody else for that matter, at best changing a few details to avoid the patent lawyer...
No variant of the 15400 (and the 15500/600's) was ever made to sew material such as denim, and only a few actually sew a 504 stitch. It''s really meant for knitwear and the like, both for seaming as well as for hemming. The 81-5 was more common and much more capable. Even the later 39200/300 etc were really not intended for heavy, hard fabrics like denim. Most of them sew it though, and like the 15000 series, it's perfectly capable to sew flies, pockets and so on, but the inseam is the real crunch. By the late 1930's Union Special had developed the 52900 (the black ones, first available in 1938) for simultaneous seaming and over-seaming with a huge number of 52900 Z variants being made during the war. Interestingly enough, these machines are a bit "rare" these days, and even the streamlined red/brown 52900's from the 1950's are not common by any standard. Very strange, because these machines are really fantastic for the purpose.
For sure, the felling machines, either off the arm ( Union Special 35700, 35800, Singer 238, 261) or up-the arm (31500 B, Singer class 19-5/9 etc, are great, but by no means the only way to go. Many manufacturers "back in the day" used flat bed twin needle chainstitch machines or even single needle chainstitch, or just used a Singer 31.
and yes... the 43200 G.. the most overhyped, overrated and blah-blah-blah machines out there. The fact is that machine wasn't used as much as some individuals make it out to be. The 11500 was far more common, and the 33500 after that. And both are far more practical too. Most smaller manufacturers or contractors simply used any single needle chainstitch or even a lockstitch to hem jeans instead of buying a dedicated machine. With unsanforised denim and cotton thread you'll get a roped hem, no matter what.
As for the Pfaff 138, what can I say? It's a very, very nice machine and it's certainly more affordable and more common than a Bernina 217, but the Pfaff has the downside of the very high shank feet with really only a zigzag foot and a straight stitch hinged foot being commonly available. Some have the dual hole presser bar so you can use regular industrial feet but most don't and those pressor bars haven't been on the market for years. Sadly enough. The Bernina 217 has the same quality as Pfaff but is far more compatible with "regular" feet, attachments and so on. Plus, the general parts availability is somewhat better. Besides that, there's very little difference in capabilities between the two machines apart from the usually wider zigzag and the definitely more practical and positive Bernina controls. I would say any tailor would be well served with either one.
