The OEM 1-ton rear axle is a semi-banjo style unit so it has drain & fill plugs but no cap like a early Dana 44 or a Chevy 12-bolt (both are much later car or light pickup axles). If your truck has a a replacement, we would need a photo of it to go on in order to ID what you have.
Light duty banjo rear axles have a slice of a center section and two axle bells that bolt up to each side of the center section. The 1-ton types have one side included at part of the center section with the other axle bell being removable and still have the axles bolted to the wheel hubs like the later heavy axles do then the wheel hubs are retained on the axle bells. The old prewar 1-tons all have a similar axle back to 1932 BB truck design but the 1938 would have shared the axle design with 1934 thru 1939 full-floating rear axles. If you have a 2-speed rear axle, then it is heavier than the 1-ton but they do have a cap on the back. They were introduced in 1938 and have a lever mechanism to select low & high speed.
