The Micas are the white-ish things at the top and bottom of the plate which hold the structure of the tube in place and against the glass. Tonally, all the mica will do is control how likely the tube is to go microphonic (which also is affected by plate size). This is mainly another comparison for reproduction accuracy.
The Mullard’s top mica, measuring 0.020″ thick, is round with 8 small ‘arms’ that hold it in place against the glass, offering 8 points of contact [editor’s note: all Mullard plates are made like this as far as I know] to stabilize the plate structure of the tube. The bottom mica, again 0.020″ thick, also has 8 little contact points, 180 degrees out of rotation from the top mica, which provides support from all sides when in the glass. In all, there are 16 points of contact with the glass, all evenly spaced around the circumference of the structure.
The reissue top mica, measuring 0.017″ thick, is a rounded square, providing 4 points of contact with the glass (the corners) the stabilize the structure. The bottom mica, measuring 0.021″ thick, is the same shape, aligned with the top mica providing another 4 points of contact, however they are directly in line with the top mica, as opposed to ‘bridging the gap’ as in the Mullard.
The main function of the micas is the support the structure of the tube and keeping it from moving around inside the glass. Clearly the original Mullard did a much better job of this, offering twice as many points of contact, all of which were evenly spaced around the perimeter of the tube. Microphonics are much more likely with the reissue due to its poor mica design.