The one at the back passenger side of the block is the tranny dipstick tube. The cap is locked in place with a pin, and there is no dipstick inside. This is where the tranny fluid level can be measured with the special shop tool dipstick. The one towards the front on the right is the engine oil.
1999 Dodge Ram 2500 Cummins Diesel
2002 Harley-Davidson Fatboy
Merlin Extralight w/ Campy Record