I have already gone through the guide on this site and my two copies look to be Sixth Gamma but they have variations. For now I will refer to them as Book 1 and Book 2.

Here is where they match:
       â—¦        Wizard logo
       â—¦        Cover art is of three adventurers fighting a large efreet
       â—¦        Endpapers and flyleaves are white
       â—¦        "ADVANCED D&D" in the angled yellow banner has been downsized, and no longer has the "D" of "ADVANCED" running off the edge of the cover
       â—¦        Wizard logo and "TSR Games" appears on the spine instead of TSR address.  Wizard logo on spine is smaller
       â—¦        ISBN now appears on spine, lower left corner of back cover, and bottom of title page
       â—¦        Title page now says "Revised Edition — December, 1979". 
       â—¦        Adds text, errata, Appendices O and P, product catalog
       â—¦        No survey form, and reference sheets are NOT perforated
       â—¦        Removes Todd Oleck artwork (pg. 40 of 5th and earlier prints) and Dave Sutherland artwork (pg. 119).
       â—¦        238 numbered pages

Here is where they don't match.

Book 1:
Size: 8 3/4"W x 11 1/4"H x 7/8"D
Heavier yellow tone in the cover. Background look more brown than black. Efreet is more orange red.
Overall cover image is shifted higher. Using Essential Reference red tag line as reference point. Text is 1 7/8" from bottom.
2011 is printed in the lower left corner on the cover in white
Spine is more vibrant due to the heavy yellow. "Dungeon Master Guide" is yellow.
Back just like the front is heavy with yellow.
ISBN numbers 1/4" from the bottom of the book.
Internal paper is rough to touch.

Book 2:
Size: 8 3/4"W x 11 1/8"H x 3/4"D
Much lighter yellow tone in the cover. Background is more black. Efreet is more red.
Overall cover image is shifted lower. Using Essential Reference red tag line as reference point. Text is 1 5/8" from bottom.
Spine color is washed due to the not enough yellow. "Dungeon Master Guide" is a pale yellow.
Back lighter in yellow but still looks good.
ISBN numbers 1/16" from the bottom of the book.
Internal paper is smooth to touch.

I understand the color differences. They obviously were not part of the same run or the printer had really bad tolerances. What get me is the size difference and the different paper. Any notice these characteristics with their Sixth Gamma.

SH60B wrote in Need Help Identifying my 1st Edition DMGs:Not to take away from this post but made me look at my first print.  The spine inlay as understand it is white not yellow or red as the description says.  Everything else seems to be the same.  See the pics.

IIRC you need to look at the top of the spine, at its edge, to see the white/red banding in the spine's binding.

grodog wrote in Need Help Identifying my 1st Edition DMGs:

SH60B wrote in Need Help Identifying my 1st Edition DMGs:Not to take away from this post but made me look at my first print.  The spine inlay as understand it is white not yellow or red as the description says.  Everything else seems to be the same.  See the pics.

IIRC you need to look at the top of the spine, at its edge, to see the white/red banding in the spine's binding.

Ahhh, I got it.  Mine is red and what appears to be yellow.