I never owned Mentzer Basic, but did see a copy once at someone else's house.I'm almost positive that it was the Moldvay edition with the Earl Otus cover, but the Mentzer edition with the Elmore cover is also very nostalgic and familiar.
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I know for a fact that my set had the lighter blue dice that I needed to fill in the number with a white crayon that was provided. Does that narrow it down?
Okay, I just looked it up on the excellent Acaeum site (https://www.acaeum.com/ddindexes/setpages/basic.html) and I'm certain I started with the Moldvay version.
The dice don't help as the Mentzer edition could come with either uncolored or colored dice.
BUT, the Mentzer edition did NOT come with the Keep on the Borderlands adventure and I am 100% certain that my first boxed set did
My first was Moldvay Basic. My copy came with KotB, but dark blue dice that needed to be crayoned. My Expert set, acquired not much later, came with light blue dice that also needed crayoning, but - I assume due to some sort of error in a factory - had 2 d10s and no d12. So my first ever separate dice purchase was of a purple d12 at the local games/hobby shop.
That Expert set came with X1 the Isle of Dread.
I still have the old dark blue d20 - with worn-away corners it is more like a ball than a die - but I think all the others are lost.