Joe, congratulations on having this great machine, an icon almost. When you used the Ivory in the washer, was the water slippery or silky feeling between your hands? It might not have been softened enough and what you were left with was a soil/mineral film on the tub. I do not know if you have a water softener, but adding a conditioner to the water with a light duty product like Ivory Snow is probably necessary for proper cleaning and soil suspension as well as for protection of the metal parts from minerals in the water. Is Ivory Snow a detergent now? I thought I read that someplace, but maybe I'm just mistaken.
The secret to keeping the aluminum tub smooth and bright is to rinse it thoroughly after washing, using a rag to add some mechanical action to the water that is spraying into the tub, remove the agitator, wipe everything dry, put a dab of grease on the top of the drive post and leave the agitator in its side in the tub so that it won't fall and break. Leave the lid open for a while to make sure the drying is complete. I remember seeing an aunt's Master with the wringer tipped in just enough so that the lid rested against it in a partially open position. Of course, there was a towel over the wringer to keep the lid from marring the paint where they touched. These machines were made for use with harsh, alkaline chemicals like washing soda with soap and later products like Tide, but the relatively short exposure to the wash water did not do real damage as long as the tub was properly rinsed and dried which is why so many of the tubs look so good even today. Any discoloration (like from really hard water) can be taken care of with a product like Bar Keepers' Friend or citric acid crystals on a damp cloth followed by a good rinsing and drying. If there is any minor surface roughness, a Brillo pad will fix it.
