The serial number is located on the flashlight's barrel near the front. Mine is a "D" series. There is also a "DL" series. This web site
gives detailed instructions for taking your Maglite apart, according to which series it is. (Scroll down or click on hotlinks for exploded drawings.) But, instructions for my Maglite said I could unscrew a retaining ring in the Switch Sub Assembly (#13) with a special tool marked in the exploded diagram as #25. I fashioned a similar tool by grinding four indentations on the end of a metal tube. All that happened was I broke some fins off of the Switch Sub Assembly. Nothing I can find indicates any such retaining ring exists and can be unscrewed on my flashlight. I decided I was on my own.