Before I start I should note that Maglite never produced a 1C or 1D light. Throughout their history they have only produced 2 through 7 cell lights. There have been some other manufacturers of flashlights that have produced 1C and 1D lights but due to the extremely low voltage (~1.5V) that a single cell alkaline battery produces this form factor was not very popular.
So how did 1C and 1D Maglites come about? Well, with the introduction of Lithium Ion batteries and LEDs in the early 2000s run time, voltage, and brightness of a single cell light were no longer an issue. A few adventurous machinists and hobbyists found ways to cut down a standard Maglite through a variety of ways to produce a more compact 1C and 1D light.
Shown below are my red 1D and black 1C lights. The 1D actually takes a 26650 Lithium Ion battery that is slightly longer than a D cell and provides 4.2V. The 1C takes an 18650 which is also slightly longer than a C cell. Both batteries require a sleeve inside the light because the diameters of the Lithium Ion batteries are narrower than D and C cells. The C cell also had its end cap machined out for the spring to result in an overall shorter light. Although the lights aren't exactly sized to a C or D cell most folks still refer to them as a 1D of 1C due to their decreased length. The serial numbers on the lights are for 2 cell models so it's easy to see that these weren't done by Maglite.
Both of these lights have been shortened differently. The 1D was shortened by cutting off the excess length on the back end of the light and the barrel was then re-threaded. As you can see the grip knurling extends all the way to the back of the light. The 1C on the other hand had a section removed from the middle of the light and it was then joined back together. The grip is actually shortened and the smooth part near the end cap remains. Shown below is a comparison photo to standard 2C and 2D lights for reference.
I run both lights with just a standard warm drop in PR based LED. They put out about 60 lumens and run for several hours. There are many brighter alternatives but I find the 60 lumens to be plenty and the extra run time is always a plus.
The 1C looks remarkably similar to a 2C Kel-Lite. Kel-Lites were the predecessor to the Maglite and they used an external switch which resulted in an overall shorter length.
These lights have been a lot less popular in recent years and there are only a handful of folks still modifying them. These were very popular modifications in the early 2000s when there weren't many options for single cell Lithium Ion LED lights. There are now countless ones that can be found for under $20 on Amazon and eBay so the time and effort required to cut down a C or D cell Maglite likely isn't worth it. They are still fun and collectible though so I'll be hanging onto mine for quite some time.
As always I'm looking to buy Vari-Beams and Vintage/Rare Mags. Please let me know if you have any, top dollar paid.
