Reservoir Mounted Sight Glass, 5 inch Inch Length, 1/2-20 Threads, Aluminum

For reservoirs with no interior access, the front mount style lets you install it from the outside as long as the tank wall thickness permits tapping. If you do have inside access, it can also be installed using the furnished nuts.

Front mount style means you can install it from the outside of the reservoir without ever needing to reach inside. That comes in real handy when the interior isn't accessible. If you do happen to have inside access, it can also go in with the furnished nuts. Either way works. There's also a 1/16 inch adjustment built into the construction so you've got a little wiggle room to get it sitting straight on the wall.