No, that was unusual, a plaster or mortar coat was more usual when leaving the chimney bare. Acidic vent moisture throught the masonry may have been trapped by a sealing paint coat to powder the plaster. Asbestos was just beginning to be used in the 30's and came as 'wool' or boards, not a panel-like cardboard at that time. Asbestos 'wool' may have been mixed with plaster. It is possible that someone thought it a good idea to use asbestos panel later, though.