Okay, according to Historical: Operation Klondike p. 156, the Clans' prototype ER small and medium lasers work the same as the Inner Sphere models we know today "with the following exceptions". The sole exception then listed for the ER small laser prototype is that it generates 2 heat per shot...which so happens to be just what its regular IS counterpart does as well, anyway. ???
So...are the two actually identical after all and it's only the Clans' early ER medium laser (with its weight increased by a half ton) that's noticeably affected by its prototype status for game and construction purposes?