I'd be interested to hear more about the form(s), too. OED2 Suppl. (1982) gives "sastruga" as the lemma, from "German sastruga, < dialect Russian zastrúga small ridge, furrow," ultimately from "strug plane." The intermediary form is a verb meaning to smooth/plane, but the web page either messes up the verb's final letter or spells it requiring a font this computer doesn't have (despite having been set up to support a variety of Unicode glyphs).