1/2 plate holders--size and modification

I have a nice set of 6 double sided plate holders, which I imagine were meant

to hold 1/2 plates, however their size does not appear to be standard.

According to the inside measurements, the plates would be 4.75 x 6.5" (120 x

165mm). The external width of the holders is 5.78" (147mm) slightly smaller

than those of a 5x7, 5.9" (150mm width). I have two questions:

 

1. What were standard plate sizes? I had, wrongly it seems, assumed that 1/2

plate glass and 1/2 plate daguerreotypes (4.25 x 5.5"), were the same...

 

2. Does anyone have any experience adapting smaller plate holders to a

modern 5x7 back? These have a 'slot' along the edge to hold them into the

back, like a graflock back. I was thinking of adding a slight amount of hard

rubber inside the slot, which would extend the width to that of a regular 5x7--

any suggestions?

Half-plate would be 4-3/4"x6-3/8", so your measurments sound about right. Whole-plate is 8-1/2"x 6-1/2". Quarter-plate is 3-1/4"x4-1/4".