Bronica ETRS Intenscreen question

I've just bought a second hand gridded bright screen for my ETRSi. Although it is not marked, I think it is a Beattie Intenscreen since it matches their 2011 catalog drawing. The grid has 6 divisions horizontally, and about 4.5 divisions vertically. However there are four corner markers on the corners of a 4 x 4 division square in the center of the screen (see image below). Although they look like the corner markers that also form a centered 4 x 4 division square on the standard ETRS grid screen, they are not in the same positions because the standard screen has 8 divisions horizontally by 6 vertically. By my calculations, and assuming 95% viewfinder coverage, they would demarcate about a 35 x 35mm square in the center of the frame. It doesn't seem to correspond to any format back that I'm aware of for the ETRS (it seems too small for a 4x4 127 back, even if there was one, which I am not sure of). And while about the right width for a 35mm frame, the height is wrong.

Does anyone know what this is for???

Thanks
Andrew