Response to Hi DJ, The difference is which heater the control works on, the 6203-071B works on the Laars ES, ESII, EPS, EPC-II, and LLD models of heaters. The 6203-071D works on the Laars EG, EPG, ESG, LLG and EPM model of heaters. You should be able to pull one of these model #'s off of your heater.

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