If you want an all metal attachment made for the old machines the closest thing is the Swiss zigzagger, it turns up now and then, but it takes time to find one at a reasonable price. In the straight stitch setting it is the same as a walking foot.
There are walking feet with metal
clap-on grip, most general low shank foot should fit on the 221.
Here's another. You need the small basic straight stitch foot to fit the old machines, not any foot will behave well. You can by a
used one, or a
new one. There are slight variations to the vintage feet and different part number, make sure they are from a straight stitch Singer model and it should work fine. Bying from sellers like Sew Classic and The Featherweight Store have an advantage, they know what type of machine you have and what they are selling.