Second try at first post, don't know if this will work...
I was at Purgatory on Saturday (Sept 22) before noon with several other birders with scopes (sorry I don't remember your names), watching the small dark gull for marks that would prove it was a young Sabine's Gull.
I've posted some flight pictures I took from the west blind area. Not great pictures, but some show the top of the wings in flight, and I'm not seeing the "M" we were looking for.
There's a chance I was looking at the wrong bird? But there seemed to be only one small dark gull on the spit. I've seen other photos on flickr (from an ebird report) from the day before that do show the marking. Love to hear from anyone that was there at the time on whether these match what they were seeing.