for the US machines of this series - 88, 89, 90 and 91 - that designation indicates the Power Source - the basic machine is the same - with the exception of some housing changes to accommodate the potted motor of the 91.
There is a plate that fits the 15 that covers the feed dogs - and you can also use the one that comes with the buttonholer (any of the straight shank style, they all fit). One other item - the bobbin case is going to be different than the US 15's of this group, as they have an 11 o'clock case finger.
Here are two feed dog cover plates. The smaller one to the left fits the Singer 66 - the one on the right fits the Singer 15.
