... But if you took one to a company that does vinyl siding, they may recognize the panel and the mfgr. that made it. Another option is to find a Roofing Supply company. A lot of times, they have Siding Supplies too, or at least may be able to help you or give you references to someone who has. My whole neighborhood was originally sided in these shingles. I happen do vinyl my own home but 3 other homeowners had professional vinyl siders do the removal and installation. I can't believe that they wouldn't know who makes what. There aren't that many mfgrs. out there!
Other than that, maybe it's written on the BACK of a panel. You may have to check a few to find one with a 'stamp' on it. How 'bout checking with your neighbors??!