I mainly collect 'non-letter serial' D cell Maglites. What does this mean? Maglites without a D in the serial number are from before 1992. These lights are quite a bit different from the later 'letter serials'. The biggest difference is the size of the light. Non-letter serials have a much thicker body and weigh much more. This gives them the feel of a much more sturdy, durable light. If you've never held one I highly recommend it versus a new Maglite as it feel like a different light entirely. See below for an example of a letter serial versus a non-letter serial.

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• Maglite D Cell O-ring, Tailcap (108-029) no D in serial number For the 2-6 Cell D Size Maglite Flashlights, no D in serial numberManufacturer part number: 108-000.
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Aug 06, 2017 TM is stamped after the Maglite logo as the trademark was claimed but not registered as shown in the above photo. Serial Range 2D 1 - 18,000 3D 1 - 22,000 4D 1 - 11,000 5D 1 - 14,000 6D 1 - 5,000 7D 1 - 4,500 1980 In 1980 the focus grip was moved to its standard location and the switch hole and cover were standardized.

Shown in the above pic is a 3D letter serial (bottom) from around 2000 and a non-letter serial 3D from around 1980 (top). The leading digit, 3, specifies the cell size in the non-letter and the leading D and 3 in the letter serial specifies the cell type and number of cells. The rest of the digits specify the number of the light in the production sequence. All lights started at number 1 and worked their way up. So, the earliest 3 cell light would be 30000001. The non-letter light on top is 30051558 so it is the 51,558th 3D cell light produced. This numbering carried over in 1992 and did not reset when the letter serials came out. Older lights did not differentiate between the cell type in the serial. Newer LED lights will also have other letters on the prefix side as well. This blog is specifically about incandescent lights.
Shown below is a new (1992+) end cap threaded into an old (1990) barrel to show the difference in size. The red barrel is an old production non-letter and the jade end cap from a new production letter serial.
The secondary update was to the switch. From 1979 until 1992 Mag Instrument used a retaining ring switch. The retaining ring was made out of aluminum early on but was changed in 1990 to plastic. The aluminum ring resulted in more difficult maintenance because the retaining ring gets stuck when dirt or corrosion get into it. The aluminum ring also resulted in higher production costs because the aluminum is more expensive to produce than the plastic. The 1992+ letter serial switches use a snap ring instead of a retaining ring which also results in less machining and a lower production cost.
Shown below is a 1979 to 1989 aluminum retaining ring switch on the left, a 1990 to 1991 plastic retaining ring switch in the center, and a 1992+ retaining clip switch on the right. The retaining clip is difficult to remove from the barrel and removal is not needed for most maintenance. It should be noted that Maglite still sells retaining ring replacement switch for older lights but they are now all of the plastic ring variety. The aluminum ring has been discontinued since 1989.

These non-letter serial lights are far less common than the later lights. A new 3D is around a 48,000,000 serial which means Maglite has produced around 48,000,000 3Ds as of this post date. Mag Instrument will actually tell you the date a light was produced based on this serial number although I have a Quick Dating Guide that will give you a rough idea without having to go through Mag.
It's interesting to note that when 1992 rolled around only 4,000,000 of each model, and sometimes many less, were produced when the lights changed to the currently available models. Currently Mag Instrument will sometimes sell 4,000,000 lights of each model a year so non-letter serials are much more rare due to their low production numbers and age.
The variations on the standard black model are also much more rare on the earlier lights. Lights came in far fewer colors and there were far fewer special editions than there are on the newer style lights. I've detailed these here.
Another easy way to tell the difference is the addition of the panther logo on the bezel of the new style lights. According to Mag Instrument a panther represents a creature of the night and was added when the models changed in 1992. Shown below is a new style letter serial with the panther logo on the bezel on the bottom and the older style bezel on top.

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Maglite Model By Serial Number