The Greist model is quite similar to the Swiss made one (Singer) and if you want a Swiss style, it can easily cost MORE than your FW (at least here in the US). I was looking on eBay, and forgot about the other style.
The earliest Singer zigzag is a side mount unit - and I've seen two different ones so far (modifcations for improvement)
then they went to a front/back mount, and the early one looks more like the Ruffler - no "cover" (this is the one posted above)
the next is the black one with the round cover, it's still rather small and will only do a basic zigzag, but you can adjust the width.
And the last US made Singer is the larger black one with the interchangeable cams
The Swiss made one is the one that comes with the white case, it's rare in the US and often sells for over $200.
However there is also the Greist units. They are almost indentical to the smaller black Singer style, and they come in both plain zig zag, and a slightly different one that has very small interchangeable cams for more decorative zigzag patterns. the cams are flat and smaller than a dime - one person described them as "ninja looking things" and that is a good description. If you look for a Greist one, you will need to get one that fits short shank side clamp machines.
With the others - I have seen the both the smaller and larger Black Singer ones that are slant shank to fit the 301, and most sellers don't know what they have, and the instructions are the same, and often say that they are for the unit that fits the both straight and slant - BUT while the instruction manual is the same, the unit is not.