Ken, Michael,
Thanks for your replies. Here are some more images, I think there are no shims,
<img src="https://farm6.staticflickr.com/5576/14915403065_ff7e2d15c1_z_d.jpg">
<img src="https://farm4.staticflickr.com/3858/14915403595_df4b7e5520_z_d.jpg">
<img src="https://farm6.staticflickr.com/5551/14915404225_4fc7cf5554_z_d.jpg">
<br>
This is a pic of another focus screen hat fits but has no split screen and so I find it very hard to focus accurately when shooting wide open. Like the split screen on, it has no "feet"
<img src="https://farm6.staticflickr.com/5596/14728742719_ea779a54a8_z_d.jpg">