FX K&L Buffer Tube Adapter for FX Panthera (seeking information)

Hi all!

I'm thinking about buying a K&L Buffer Tube Adapter for the Fx Panthera but before I take the plunge I'd like to gather some information about this accessory.
Is the size Mil-Spec or Commercial-Spec? The official website doesn't specify.
Does it fit both Rifle and Carbine-length buffer tubes?
What brands of buffer tubes and stocks do you recommend for both types of stock receiver?
Any additional information you can/want to provide would be greatly appreciated.

Regards

P.S. Since English is not my mother language I use a translator. However, I hope you will understand what I am writing.
 
Hi, it's Mil spec. There are a couple.of reviews on it if you use the search function.
Hi Rob, thank you very much for your reply. It is very helpful. The reviews you are referring to are on this forum? Do you have an FX Panthera with that adapter? If so, would it be too much to ask you to post a picture of your airgun?