Dear Joop, in the 1977, when appeared the M4-2, the Leitz company was need to reduce the very high building cost of the previous M4 and M5 cameras. The cost cutting involved the elimination of the self timer, the use of the stainless steel to replace the chrome brass for the top and bottom cover and the redesign of the finder, in respect to the previous M4/M5: was eliminated an optical glass element, while another glass element was replaced by an acrilyc element. In practical therms, the functionality remain the same, but this finder was more prone to the flare, always in respect to the previous Leica M cameras.
The M4-2 finder type was used, also, in the M4-P, all the M6 models and the first series of the M7.
In the MP is back the chrome brass for the top and bottom cover and the finder is again with the same number of the elements as M4/M5 (all in optical glass). This new finder was mountend also in the M7 cameras, from the 2885 matriculation number.
More info in the Erwin Puts web site.
Ciao.
Vincenzo