Best illustration* of what I mean that I could find.
I KNOW I've seen sack holders that had an upright, with a pair of angled struts on the bottom to support it, and another above. (They were metal; these would be wooden, so probably would have a smaller post on each front corner.) A gunnysack was hung over the upper struts U lipped over the rear upright joint; the bottom seam rested on the floor.
One person stood at the 'open' end of the V-base, and held the top-front of the sack taut, while it was being filled, then the top of the sack was flipped off the support, and tied or sewn shut.
The floor notches would keep the stand from both "walking" as sacks were filled; and from splaying.
The legs of the V notches in the floor (judging from the man's lower legs/lower arms) appear to be a popular guesstimate of a cubit.
The way they are placed, it looks like it would be convenient for one person to stand between the apexes, and verify how much was in each, as they were filled. The offset would keep said overseer out of the way of the fillers.
*It is an Indian hand operated, portable grain cleaner.