Yes the adapter mounts directly to the 21" driver cut out. Tested it with the 5100 and it worked like a champ. Almost exactly as modeled.18 LW2500 models terrific in this horn. I was very happy to see. This is the 1watt(2.83 vs 2v) Almost as sensitive. Probably a variable in there that I missed but they are close enough for me. The 8ohm 2500 will take 97v while the 21 will do 127 I believe. About 5dbs more but I can buy a pair of these 18's for the price of the single 21. Maybe down the road when I have more money I can buy the 21's.
I only wonder does the adapter plate have the 18 at the same throat entrance as the 21? Confusing question to ask.
Does the plate go inside the whole of the 21 opening so the 18 is exactly where the 21 would be? Does this make sense?
Sorry the attachment has the 152 in bold and the 2500 in gray. The 2500 has 290gr Mms, 30 Bl and 14mm Xmax. SO a hefty improvement over the 2400 and all for the same price.