Photo above (compliments of Mark Hester) is an original wood packing strip from an MGA windscreen lower frame rail. This one is about 4mm (5/32-inch) thick and 18cm long, but it is broken at one end. Original length should be 40 inches (1 meter) to span the distance between the legs of the lower corner brackets.

Various replacement windscreen glass is not the same size around the perimeter. All of them that I have seen in recent years have some relief of the edges for a few inches back from the corners on all sides. But they are apparently not all the same overall height.

There is a factory Confidential Service Memorandum MG/398 dated 2 July 1962. This mentions replacing the early wood packing piece inside the bottom frame rail with a newer replacement rubber packing piece. This part was used to support the original glass (which was straight all the way along the edges) so that it did not rest on the bottom corner brackets, which is known to cause a triangular break across the lower corners of the glass. The glass I have will not fit into the frame if there is any packing piece used inside the bottom rail, so it appears that the height of the glass was increased slightly along with adding the relief in the corners such that the glass is now intended to sit directly on the bottom frame rail with only the soft rubber glazing strip.

On the other hand, I have a report (December 2001) of installation of one of the thin glass windscreens (maker unknown at the moment) which would move vertically in the frame. When it settled to the bottom it left a gap above the glass at the top corners of the frame (where there is an edge relief in the glass) that you could slip a knife blade through. This one may have been made to the original overall height dimension, in which case it probably does need the original type support packing piece inside the bottom frame rail. (But for the life of me I still don't understand why anyone makes the glass only 3/16 inch thick).

This packing strip is not shown in the Service Parts List, and is most often overlooked by the replacement parts suppliers. However, on 7/27/2014, Neil Burnside wrote:
"I bought a Brown and Gammons windscreen seal kit, and it cam with the rubber packer in it".