Looking for a replacement derailleur hanger, How to be sure which one will fit, there are no markings at all on the part? I found something looking very close at derailleurhanger.com

You might try the *contact us* link on their site. See if you can send *derailleurhanger* an image, with a ruler in the photo, for scale. I’d also include as much info about the origin of frame as possible.

Last, I’d order two and keep one on the shelf.

That’ll work. #27 is one of the most common hangers usually found on hybrid type bikes. So common that even a few different manufacturers use the same number. Stock hangers are usually lower quality sand casted hangers, aftermarket ones are like the one you found is machined. If it doesn’t matter to you, you can find cheaper casted versions for around $6-10.