Hi Ipcress, I have seen similar red marks on Shida porcelain, and it seems that the enamel decoratio on them was made in Arita. But I have done some searching today and think now that this exact mark is related to a certain Arita kiln, that was active from the Edo period into the Meiji period. I will try to find out more about it. I have a plate (see the pictures below) with this mark myself, which I always believed to be Shida, as it shows certain characteristics of that ware, but it seems I was wrong. The other imari plate has an unusual mark, I tend to date it to the late Edo period, but it could be later of course...
Mat