Nice article with pictures on the SKS prototypes.
https://translate.googleusercontent.com/translate_c?act=url&depth=1&hl=en&ie=UTF8&prev=_t&rurl=translate.google.com&sl=auto&sp=nmt4&tl=en&u=https://www.kalashnikov.ru/sks-kotoryj-tak-i-ne-vstupil-v-boj/&xid=17259,15700022,15700124,15700149,15700186,15700191,15700201&usg=ALkJrhi5QuMM94jvPyVMLK-W7TiFH9cmBQ
This looks to be an "SCS-31" which fired x53R rounds. Was this a predecessor prototype to the x39 SKS?
Yes, the x54R round proved too powerful for a light carbine (as is evidence in the OP's link). in ~'43 the x39 round was decided upon and the competition to build a carbine for this round began. Simonov had a leg up on certain other designers for a fixed magazine semi-auto version (including Kalashnikov). The rest is history!
See this great writeup posted by Bunker for some designs between the time of the OP's link and the SKS-45 we all know and love: https://clone.sks-files.com/index.php?topic=4105.0