Any info on fat bottle Ei tubes?

I saw a ad on Ebay for quads of Ei fat bottle tubes. The ad stated more output power. When I inquired as to what made these tubes have more output I was told that the bottle design had something to do with it. My question to old tubeheads is: Can a tube be made to put out more than it's rated power(EL34)? Also what would a fat bottle design have to do with power? It should be a internal design that would make all the difference no? Just curious as I am somewhat new to rolling of those magic bottles. Thanks,
Have not used em but heard they are rugged and maybe little bright to some people (I never know how much to put into such reports). My understanding is that most Ei's (Yugo/Serbian)floating around now are not actually EL34's but a true beam tetrode and more rugged. Ei made a EL34 but this tube is not it. Sound dif. Many of the 6CA7s were relabled by the distributor (Orion?)Think I read about it at Jim's Citation Amp and Vacuum Tube Page. Try there for more info, check it out before you buy.
Thanks Clueless for the info. The person selling these Ei's states he has a lot of them, so maybe they are relabled. I have enough EL34's presently but was curious as to what others might know. Good listening.
FYI Triode Electronics sells the same tubes Ei Fat Bottles for the same price as the ebay dutch auction. They are sold as matched quads and come with a 6 month exchange warranty. I have bought many tubes from them over the past year and have been happy with their service. I try to use the dealers before ebay because down the road I need them to be there (I own 10 pieces of tube equipment). I am not related to the above dealer. Hope this helps.
Hello Khawk2 and thank you. The triode site is great! Lots of info for the newbie and the vet. As you said you want someone to be there for future sevice and that's important to me.