This IR cut filter is an accessory for your filter wheel or along image train 
it is not inside the 2600MC as I’ve pulled my camera apart for cleaning . The IR cut protective window inside 2600MC is rectangular , same as sensor
It may very well be same glass material but not confirmed 
Description on camera for protective window is D60-2 IR Cut , description for this round filter accessory is D60 IR Cut , obviously different 
Still need confirmation by ZWO
 
Attention : ZWO 
Please provide answer on this post 
Thanks in advance