Quick question about NYAR MP15s

I noticed that all the mp15s except 151 have the lower rear window on the conductor side plated over. Why are they plated over and why didn't they do 151? (unless they did and i'm looking at old photos)
#151

#155 to the right (in the original picture you can see the number board I don't know why it got cut off)

#156

#159

When that window breaks, it's cheaper to plate over that window than to purchase the glass, which is quite expensive. All things considered that window does serve much of a purpose anyway.. most other railroads with end cab units have done the same, including LI.

I figured it had something to do with broken glass, the only reason i asked is because i have the NYA Athearn Genesis #156 and want to add as much detail to it as i can (working ditch lights, marker lights etc) and thats when i noticed the windows, and if i can find another one either 151 or 159 i wasnt sure if there was plated window on 151

As mentioned, that window serves absolutely no purpose anyway, and those that still have it would be so dirty you wouldn't be able to look through it anyway.

To my knowledge, three LI units also have that window plated over: 154, 166, and 171. I know a couple of the SW1001s have it too, not sure which.