Marked 20-sided Die

TSR's original dice set was included in Holmes Basic, Gamma World and sold separately

In the 1970s, rolling 1-20 wasn't as straightforward as today. The original dice set available from TSR included a white 20-sided dice, but it was numbered (and pre-inked) with 0-9 twice rather than 1-20. So these dice were actually d10s, and were most easily used to generate percentiles by rolling the same die twice in row or by rolling two different colored dice together. TSR even sold a separate white and pink set of Percentile Generators.

But since the beginning D&D has always needed d20s, for attacks and saving throws. The earliest D&D rulebooks don't explain how to use the 10-sided die to generate 1-20, but by the time of Holmes Basic, there was a recognized need to explain this, as the rulebook teaches two different ways to roll 1-20 with these dice. One is near the end of the book in the section "Using the Dice", which isn't in the Holmes manuscript, and so was added by TSR. This method uses a secondary "control" die to determine if the number is 1-10 or 11-20:

"For example: to generate 1-20, roll the 20-sided die and 6-sided die, and if the 6-sided die comes up 1-3 , the number shown on the 20-sider is 1-10 (1-0), and if the 6-sider comes up 4-6, add 10 to the 20-sided die and its numbers become 11-20 (1-0)".

The other method is described in the main part of the text in the section on Saving Throws (page 14). This was written by Holmes as it is found word-for-word in the manuscript:

"Numbers can be generated as follows: Mark one set of faces on a 20-sided die by coloring with a red permanent marker on one of each faces — 0, 1, 2, 3, etc. The marked faces will be considered to have a ten added to them — 1 = 11, 2 = 12, 3 = 13, etc. Unmarked

0 = 10, marked 0 = 20. This die will also be used to determine the results of combat from the combat table."

The picture at the top of this post (from an Ebay auction that indicated the dice were from a Basic set) shows an example of this: the owner has colored half of the faces of the 20-sider in a red color. The white faces represent 1-10, and the red faces represent 11-20. As I mentioned above these dice were pre-inked, so one couldn't just color the two sets of 0-9 with different colored crayons, the faces had to be marked to differentiate them.

This method is referenced again in the section "Combat Melee":

"The probability of a hit is converted into a random number of 1 to 20 (the specially marked die is recommended)" (page 18) and "A 20-sided die must be marked or colored so that
one set of sides 0-9 is different from the other set. Count 0 as a 10. The marked set is then read as if 10 had been added to the roll (11-20), treating 0 as 10 or 20. This die is used for all combat resolution" (page 19)

Holmes probably learned this marked die method from other gamers, as there are earlier examples of it. For example, below is an auction photo from last year, for an auction you may have heard about, an original woodgrain D&D set that sold for over $20,000. Included with the set in the auction was a 20-sided die and a note (with the date of ~1974 given by the auctioneer). In the note we see similar instructions, with the white half of the die being 1-10 and the orange half being 11-20.

In 1979, the 1st edition DMG still assumes use of these 20-sided d10s in the section "Dice", on page 10:

"If a d20 is used either 1-20 (assuming the use of a standard d20 which is numbered 0-9 twice without coloring one set of faces to indicate that those faces have 10 added to the number appearing) or 1-40 (assuming that one set of faces is colored) can be gotten by adding 0 if 1 or 2 is rolled on the d4 and 10 or 20 (depending on the die type) if a 3 or 4 is rolled"

The structure of this sentence is complicated, but Gygax is saying to use d4 control dice to turn 1-10 into 1-20 (for an unmarked die) or 1-20 into 1-40 (for a marked die).

This was a short-lived era as other manufacturers began cranking out dice. 

At some point (I don't have a date but will update this post if I find it), 20-sided dice that were not pre-inked appeared, which allowed for coloring the two sets of numbers with different colored crayons. You still had to remember which color was low (1-10) and which was high (11-20). I have a dice like this that I received in an auction a while back (I can't even remember what it came with):

I also don't know when the first 20-sided dice that was numbered 1-20 first appeared, but the standard d10 appeared around 1980, possibly debuting at Gen Con that year. 

In the last printing of the Holmes Basic rulebook, dated Dec 1979 but certainly from 1980 as it is the third version with that date, the section on "Using the Dice" was revised to refer to "the assortment of 4-, 6-, 8-, 10-, 12- and 20-sided dice" (page 46), and the portion about the control die no longer refers to 1-20. Holmes' instructions for making a marked d20 is still found in the section on Saving Throws, however.

The Acaeum reports that some sets of Holmes Basic include a set of six dice. I've never actually seen one of these sets. It does seem strange TSR would revise the rulebook to refer to the 10-sided die without actually including it. But I'd like to see it confirmed that a set shipped this way versus having dice added later. A complicating factor is that Holmes Basic set was sold up until at least 1986 (I have a catalog from then listing it), so some may have had 6-dice sets added to them at later date.

Certainly by the time of the Moldvay Basic set and Dragon Dice, both from 1981, we have the standard 1980s set of six dice, including both the 10-sided die and the 20-sided die numbered 1-20.

See also:
Veteran of the Dice Wars
TSR Ads in Boys Life 1977-1982

And Jon Peterson's articles on the history of dice in D&D: 
How Gaming Got Its Dice
The Origins of Dice Notation