manicmike

No worries. I'd actually thought you'd assumed (as I did) that all 15s are the same. Every single one I've ever had has been left to right, but I've never had a 15-88 to -91. All other models are left-right like the 66 (and 99, 27, 28 etc etc) or front to back (206, 306, 319, 320, 400 series and more).
It's one of those things we learn by experience but there's no easily found definitive list. Here's what I have so far:
Singer models that thread R --> L: 15-88, 15-89, 15-90, 15-91, 201, 221, 222
My new (!) Improved Family or 15-1 is left to right.