It is truthfully hard to say.
1.) There were many 88's that were still in the original form that were still in use, as well as many G88/S rifles that were never modified and they could not use the cover, because it blocked the packet clip from dropping out the bottem after the last round was ejected.
2.) Supposedly these covers were unpopular, although personally I don't understand why. They are not easily taken off, so they should have held on reliably, and they keep mud and debris out of the rifle. Can't remember where I read that one.
3.) Author Hans Dieter Gotz in his book German Army Rifles & Machine Pistols 1871-1945, put total production of the magazine covers at 75K pieces. Considering there were around 500,000 88's still in use by the Alte Armee, you just can not say for sure.