Hi, Rycen; This will be a long post:
The "heavy duty" and "standard" variations only applied to "C-cell" models. The "standard" model had thinner aluminum walls than the "D-cell" flashlights. It did not have a spare lamp in the end cap (a end cap that could hold a spare lamp could be purchased separately, as well as a "yawarwa" [sp] end cap, intended to be used with the smaller "C-cell" lights, making it a martial arts weapon), only the simple black plastic "push" switch that broke fairly easily,
and the "small" head (more on head sizes later).
The "heavy duty" model came with the new 3-position switch (rear was "off"; middle was "flash"; front was "on"), spare bulb end cap, aluminum walls the same thickness as the "D-cell" models, and a medium size head.
The standard "C-cell" models were introduced during the mid-1970's (I believe; that's when I first saw them), and the heavy duty models came out in the late-70's. I was my guess that the heavy duty models would replace the standard models, but that never happened due to the introduction of the Mag-Lite (the downfall of the Kel-lite).
If you can "hang around" awhile, I'll go ahead and give some more information:
The first Kel-Lites were "D-cell" (2 through 7). As far as I know, these were the first heavy duty flashlights designed specifically for the law enforcement and security fields.
Other flashlights would come along (B-Lite, Pro-Lite, etc.), but the Kel-Lite was "king" (the "Code-4" flashlight came close to dethroning the Kel-Lite, however).
When you bought the flashlight, you picked either a large head or small head model, depending on what kind of beam you wanted:
A large head gave a "flood" type beam that didn't travel very far (later versions gave longer throws); A small head gave a narrow long throw beam. In the mid-to-late 70's, a medium head was introduced to "split the difference" between the large and small heads. It should be noted that many Police/Sheriff Departments had weight and size regulations, and this determined what configuration was purchased.
All the flashlights came with the plastic "push" switch, and a flat end cap without a spare bulb. Later models came with a "store-a-bulb" end caps, but they were still flat. These were replaced with rounded end caps. All standard "C-cell" models came with the flat end caps; rounded ones came on the heavy duty models.
In the late-70's, the 3-position switch was introduced. As I said earlier, the plastic ones broke fairly easily (I knew a Deputy who, after breaking 3 switches, replaced the switch with rubber washers); this new switch was mostly metal. The switch had a polished push button, with a red (sometimes black) plastic insert; this insert is pushed when the button is in the center ("flash") position. In my experience, these switches broke even easier than the plastic ones; two days after I purchased a 5-C heavy duty light, the retainer that held the spring under the plastic insert broke, sending the spring and insert flying across the room. I also have a 4-D light with a broken switch. On the other hand, I know of other Kel-Lite owners with this type of switch that have never had a problem. Perhaps the first production lots were bad.
When Mag-Lite was introduced (I don't know if this is true, but I heard that Donald Keller [inventer of the Kel-Lite] designed the Mag-Lite; I also heard he designed the ASP Tac-Lite/Triad), it had design advantages over the Kel-Lite that made it "king": recessed switch button that didn't break when dropped; and a spot-to-flood beam that elimated the need to switch heads. The switch design of the Mag-Lite does make the flashlight "longer" than a Kel-Lite (i.e. a 4-D Kel-Lite is roughly the same size as a 3-D Mag-Lite, etc.)
That's about it regarding Kel-Lites (probably more that you wanted to know). I will end now with a description of a "unusual" Kel-Lite (?).
Around 1976, I saw a Sheriff's Sergeant with a metallic green color aluminum flashlight. It was a 3-D cell light, with a small head, a white plastic push switch and a flat end cap with no spare bulb. Except for the color, it was an exact match for a 3-D Kel-Lite. The end cap was not stamped "Kel-Lite Industries", but was stamped "Greenwood Uniforms, Inc." I don't know anything more about the light, and it was the only one I ever saw.
Hope this helps; I now have to give my keyboard a rest.
Best regards
Mike 161