This freewheel is marked "Shimano 6D." Does anyone know which remover is used to remove it? I went to my LBS; they have approx. 20 freewheel and cassette removers. They don't have a match and did not have any info for me.
Does "Shimano 6D" ring any bells?
thanks.

Looks like you might have a Uniglide cassette hub there. Can you take a picture of the hub through the spokes?

Yep, a Uniglide freehub.

You'll need two chain whips, and just unscrew the small sprocket, and the rest of the sprockets should slide off.

Look for spares on E-Bay.

Is this associated with a specific group set like Shimano 600 or 105, etc? Trying to do more research on it.

So the first cog should come off clock-wise?

Counter-clockwise, with respect to the other cogs.

Uniglide could be found at most levels before it was supplanted by Hyperglide.

Ditto. Uniglide hubs were available in every grade from Altus to Dura-Ace. I guess that's a lower-line hub because it has black cogs. If you can find markings on the barrel of the hub (something like "FH-100") we may be able to make a better guess.