There are 2 main mfg's of water heaters. Each carries both pilot (with reignitor) and Direst Spark Ignition (DSI) models. All of them can be obtained with both gas and electric variations. Your 77 rig could have anyone of those variations. So, would you please go outside and open the water heater door and see if you can find the model number. For a Surburban unit, it would be something like SW6P, SW6D, SW6PE, SW6DE, etc. For a Atwood unit, it would be something like G6A-7, G6A-7P, G6A-8E, etc.