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					Originally Posted by Geoff Winnington-Ball  The rectangular hatch was in place from inception of the CMP to 1944, when it was gradually superceded by the round hatch. | 
	
 Actually, the 
square roof hatch was optional on all models. I recon in practice it was fitted as standard to all mid-war production CMPs, and it was superceded by the round one in 1944/1945 production. It was not so much of a hatch, more a round hole with canvas cover.
HTH,
Hanno
 
 
Source: 
Interesting pic of CMP