The Barton is basicly a Thoroughbred B with double L2 cache and 333mhz FSB. The Thoroughbred B has 256k L2 cache the Barton has 512k. The Barton core (at the down side of the chip) is 101 mm2 big, while the Thoroughbred is only 84 mm2, this is cause the extra L2 cache sits there.

It's very possible that you have a 2600+ with 266FSB or 333mhz (without having a Barton core)
How can you see what core you have... check your core: it should read AXDA2600DKV4D
if it's a Barton. The Model number id for a 2600+ 333Mhz Barton would be Model 10
You can see that in a program like WCPUID.
If it reads Model 8, you have a Thorougbred B 333mhz