The Extra 260 was basically a Extra 230 with a 260 HP Lycoming 540 AEIO in it. Only a few were actually built and it was never certified. They are all essentially experimental planes.
The Extra 300 is fully certified and has a 300 HP engine in it.

Upon further research, it seems like only six full scale Extra 260 were ever built. They were essentially uncertified prototypes and need a special waiver in order to fly in airshows.

Google "Walter Extra" if you want to find out info without getting a ton of model airplane hits.

Here's Walter Extra himself with the person who owned 1/6 of all the Extra 260's ever built, Patty Wagstaff.