By topplate I presume you mean the snout on the driver that the horn bolts to. You simply unscrew the snout. Might be harder than it seems, it can take ALOT of pussy to remove the snout. I bought a pair of 2427s, same driver but with a different snout, goes out to a 2" opening. Had to put the snout end of the driver in a large vice and remove the driver from it with a 36" pipe wrench and I needed the 36. And I'd already spritzed the thing with Kro-oil. No kidding. The stainless threads of the driver gall the aluminum threads of the snout. The indentations on the driver cover were perfect for the wrench to bite on, did the job without a scratch.