1863 Scott 4 6p rose
Here is my Scott 4 6p rose with rough perf (CV $20).
Looks a lot like yours. Left: wmk 5 "Small Star"; Right: wmk 6 "Large Star"
Here is the small star watermark on left.
You think you have a 5a 6p dull red @ CV $275?
Remember, common stamps occur commonly, rarer ones not so much.
These stamps come in shades according to SG. You need to find a small star-sideways. By the way, the Scott 4 6p rose can also be found with small star-sideways.