1955 D LWC MM ?

I don't know, but we will see.

After looking, I think it is unlisted.

I never advocate cleaning and it looks like you follow a good practice of not disturbing your coins but the grime packed in the mintmark could be covering helpful information. As is I am seeing, on the inside of the D,there is something at the top left and at the bottom right .
The one at the top seems to represent a southeast movement for a rpm but the one at bottom would represent a northwest movement for a rpm. For this to happen and be a rpm it would need to be a D/D/D. Certainly posible but with the wear and packed grime I can not say for sure

As I said I do not advocate cleaning but in cases like this I do remove the grime. What I do is soak the coin in Naptha for a bit then use a sharpened bamboo point and disturb the grime a bit. The use a soft artists brush soaked in Naphtha to wipe away the grime , and pat dry.

Looks like an RPM, but not sure wich one...

This one does look like a RPM. I find no match on any of my sources so far. But I would recommend removing the grit around the devices. I would use a mineral oil soak for a while to soften up the grit. But carefully to keep any sand in the grit to keep from scratching the fields of your coin. It would be nice to see what is going on in the center. Here is the method I use:

This will only remove grit around the devices. (not to be used on BU/Proof coins as the method will scratch the surface of these coins. This will not remove discoloration, finger prints, carbon spots, and other things like these. This is just to remove the grit from around the devices. Use a soft green thorn to remove the grit carefully as it may contain sand. No toothpicks, or hard woods as they will make the grit scratch the surface of the coin. (practice on a normal gitty coin to get the technique down first) If needed used the soft tooth brush maybe a second time to remove unwanted grit. Best to have the techinque down first before attempting your coin. It may turn out to be a die chip or a RPM. But it is interesting to look at.

I just pat dry by pushing downwards several times. with a facial tissue. The excess will help to protect the surface. Because it is similar to what is used during the minting process it not hurt the coin. (olive oil will rot the coin as it is vegetable based)

It is definitely not 1955D-1MM-010 The location is different. Looks like a curve to the east on the RPM at the top. Is that what I'm seeing east of the obvious? The bottom look looks more like a coin scraped that area. The metal curving into the mint mark. Looks like a SE RPM?

To my eye, it looks like some of the MM metal might have been pushed into the field.