Have you ever been to where central american milksnakes live? I haven't produced 1000's of snakes like some here. Still, I have never had a getula or Mexican locality milksnake not produce when cooled to around 60 deg. And then I cool them at night only. Occasionally night temps dipped as low as 53 deg, but only for the night. Heat during the day offered on one end of the cage. They produce.
My guess is this constant 55 deg thing is a generalization that happens to work most of the time. I also think you may be holding to those temps because that range has become ingrained in the way we think about breeding colubrids. I truly believe that snakes will not choose temps that impede their reproductive success when given a choice. Better you test this yourself than let me tell you. I'm can only give my experiences that lead me to believe that constant 55 deg is not magic.
BTW, I know of a breeder that produced ruthveni, and he cooled them to no less than the low 60's. Ruthveni are high elevation kings in an area with a cooler climate than Central America. Even if this was just a few cases, it does show that 55 deg is not necessary when other requirements are met.
Test it.
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Mark