There was a TSB issued in late '57 that mentions a production change to the model-year 1958 261 block and head gasket, specifically to reduce coolant flow at the front of the head and improve it at the rear. The change was stated to improve temperature distribution through the block and head, resulting in improved component life. (The change has nothing to do with locale or seasonal changes.)
The TSB details updates for earlier model-year 261 engines. One of the rear-most cooling passage openings in the block gets drilled out to 3/4", and the alternative gasket that restricts flow to the front of the head is specified.
The two images show the TSB as issued December 9, 1957.
It was by chance that I recently saw this TSB. I picked up a set of original copies of the late '57-'62 TSBs and '59-'61 Service News. Paging through gives a pretty interesting view of what was going on in the service departments in that era. Starting in '58, a dominant theme in the TSBs was servicing the Turboglide transmission, and then in late '59 through '61, a ton of material for Corvair service and customer complaint resolutions.
