Another pic.

r u referring to the brass piece that sticks out the right side? That is the choke shaft. Of course, yours doesn't need it - it already has one on the aluminum piece that holds the air filter.

It's a WT 215. It looks like it might work OK. I think the carb just breaks loose from the air filter cover - is that right? Then one uses the choke shaft lever instead of the lever that's attached to the air filter cover?

yes, it will work. many carbs will work so long as it produces the correct air\fuel mix. I use zama C1Q on mine. I tried the one u have (without the choke) but mine ran eratic because it doesn't pressurize the fuel tank. (You will need a diaphragm or just a hole in your fuel cap for it to work.)
Break loose?? The aluminum air filter holder is a seperate piece if thats what u r asking.
Problem is that in order to use the original choke, u need to remove this choke cuz the original wont close with this choke open -BUT- then the shaft pops out then u need to plug the hole...

Maybe I should just return it and get a real WT-38-1

Could it be that Walbro dosen't make the original WT-38-1 anymore? Anyone else have any input into adapting the WT-215 to the Stihl FS-61?

The screw holes are standard. When u take the carb off u will see a small hole below the air intake. that is used if u use a carb that pressurizes the tank like the carb i used. With the carbs u r looking at, NO, u dont need to adapt anything to fit a different carb.

ps:
I went down your road with my 65 and tried walbro, zamas, and orig TK. Trust me - get the C1Q (or any type) with 2 fuel adj screws. When u get either the wt 38 or 215 running, u cant run it upside down(edging) , store it verticaly, and your trimmer will be moody with all 3 brands because u r at the mercy of gravity feed with this carb, also gas will splash (drool) out the gas cap bcause without pressure u need to drill a hole in the cap unless yours has a modified cap. (The store only sells gas caps with no hole) I took a echo gas cap (which fit) and tried drill a hole and add a diaphragm to eliminate the leak but it still ran moody. I did this to all of my 65s and 80s. This gives u an idea of what i went through playing with my 65. But hey ill be glad to help whichever route u take.

Here is a link that might be useful: Here is one of them