I am not sure about the "differences" between 1975-1978 models but there was a difference in the front sight between 6" bbls and shorter ones and the non-red insert ramped front sight is what I have seen on the shorter barreled ones. All I have ever seen on a 6" bbl. model is a Patridge-type front sight but they might have come with other ones also.

Thanks for the compliment on my Model 27 and I am looking forward to seeing yours.