Well, I'm not the greatest at it, but an AC in the model number is a model 15. My treadle is also an AC #. I'm going to say she is a 15-90 by the slope. Another site I read said to look at the top of the machine. If the slope was gentle from the thread spool to the tension and you could ski it and make a jump off the end, it was a 15-91. But if you tried to ski it and you ran into a wall and crashed, it was a 15-90. I think you have the crashing wall design -- hehe.
Regardless, she is very pretty. Enjoy your new sewing partner.