1981-D Washington Quarter - Filled D Mint Mark

Hi everyone! I'm new to coin collecting and recently found this 1981-D Washington quarter while going through some change.
The first thing that caught my attention was the D mint mark, which appears partially filled or blob-like compared to another 1981-D quarter I have for comparison.
Here are a few observations:
The coin weighs approximately 5.6 g (my comparison quarter weighs about 5.7 g on the same scale).
The edge shows some copper, but it appears to be from normal circulation wear.
I've included close-up photos of the mint mark, full obverse and reverse photos, an edge photo, a side-by-side comparison with another 1981-D quarter, and the weight.
My questions are:
Does this look like a genuine filled D mint mark, or is it more likely the result of wear or damage?
Is this a known variety or common occurrence on 1981-D quarters?
If it is a filled mint mark, is it considered collectible or does it add any value?
Thank you for taking a look! I appreciate any opinions or advice.