Diaphragm, low pressure, for pumps with a 2ba earthing screw

This diaphragm has a total stem length, from behind the diaphragm to the tip of the stem, of about 2 7/8". There is no steel plate between the rubber diaphragm and the cast iron armature disc.
There is no circular spigot or collar on the upper face of the armature disc. The upper face is flat.