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I am trying to find out the significance of 3 marks that appear on the back of base plates.

There are 3 circles in a row. The first contains numbers 0-9 in increments of one with a small arrow pointing to a number. The second circle contains numbers 0-50 in increments of 10 with the same kind of arrow. And the third circle contains an icon that looks like a dog tag.

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I think that this is a production code similar to the seal codes on other sets (e.g. 24R5 for the 24th week of 2015), and you need to add the number from the first dial to the number of the second dial to get a week number, and the Q "dog tag" indicates the production facility this came from. It's missing the production year though, perhaps that was not needed or has been encoded somewhere else on the baseplate.

UPDATE: The year of production is printed in the center of the single digit dial ("11" in your case) with one digit on each side of the arrow. So your base plate was made in 19Q11, which is the 19th week of 2011. This code seems to be on other LEGO elements as well - for comparison here is a picture of a LEGO Mindstorms EV3 tray from set 45544. It also features a Q, and has a "15" printed in the center of the single digit dial (the full code is 17Q15):

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  • Thanks - makes sense. Any clue on when this marking system was started as my plates do not have these marks. – Jamie Apr 3 '16 at 12:33
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At first, these look like regular old LEGO IDs. But they can't be...

These include:

  • Part ID: The type of part it is
  • Mould ID: The Mould that printed it
  • Copyright: The LEGO copyright statement.

Obviously, there is no copyright statement in this piece, so we'll have to rule that out as a possibility for the "dogtag."

We might be out of ideas as far as lego meaning here...

But Wait! Lots of plastic products have these "clocks" on the bottom!

Check out this link on SE: Seasoned Advice!

This is the date the item was manufactured. The upper dial gives the year, the middle one, the month.

Perhaps this isn't particularly correct, because of the difference in increments, but we might be a little closer. Is it possible that LEGO is just following the plastic printing norm?

Or it possible that these numbers are made to be combined? Maybe these indicate a year or some other time, but maybe...

Maybe they're related directly to the plate piece ID somehow!

As for the 'tag, your guess is as good as mine.

But here's a thought:

The bottom indicator could be a mold identifier, and in some cases, it might be a shift indicator

Perhaps this is some sort of grahpic ID instead of a number that indicates something about that kind of mold, or even something about LEGO itself.

I spent maybe 5 minutes staring at my baseplate collection, and they all seem to have one, including the vintage models, so I'm going to lean towards a "kind of mold" instead of "a particular mold."

So here's the TLDR.

Though the internet and my LEGO friends don't know, I think we can narrow the possibilities down considerably:

  • The markings mean something about when the piece was made.
  • At least one of the markings means something about where the piece was made.
  • The markings mean something about what the piece is (ID.)

The fun is in the search!

  • Thanks so much, Caleb. I have plates without these symbols that I am trying to sell and someone asked me if my plates have these marks. Mine do not and I am trying to figure out if I have early pieces or why they are missing these symbols. I'm just a mom cleaning out the basement and thought I'd check with some experts. :) I appreciate your reply. – Jamie Apr 2 '16 at 18:08
  • Since you're new, I'd like to mention the little green arrow by the side of the top of my answer, and how, when you press it, you mark this answer as accepted. Also, the arrow pointing upwards on that will decide whether you consider this response good enough to "vote up." – Caleb Woodman Apr 2 '16 at 18:09

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