It would be a good guess if you went to a local Mazda dealer and bought a battery it would be legal! 
I believe I ordered my last one from Sams Club...Interstate battery which is the correct size.
I believe the theory is you cant use a larger battery which might add 10lbs of ballast in an area you cant add ballast. Bottom line for a NB car you dont want that extra 10lbs in your trunk!
Actually, NO...
Mazda doesn't make or sell the correct OE battery any more. The original was a fiberglass wrap battery that lasted years and years. The replacements are lead acid cells, which is a completely different technology. That seems pretty non similar.
It doesn't matter what we theorize here, the rules are clear.... that they are ambiguous. There is no technical glossary entry for "similar".
TECH and everyone should read the rule. I saw they weighed batteries at the runoffs as well last year. I hope this isn't SCCA tech trying to play gotcha with an open ended spec like they have so many times before, especially in this class.
The following incident at miller has changed the way we handle the battery rule. I got hit hard in the right rear from the side as I was mostly around one of the sweepers, counter-steered and went off with tires digging in with the sidewalls. The hit crushed the fenderwell, crushed the battery, popped one of the caps off the battery, and the way the car stopped, threw acid EVERYWHERE in the trunk! It looked like a fire and the fumes were really bad too.
Anyone in my customer base that saw that damage now have fiberglass wrap batteries, and the rules allow that. Anything as small as the OEM size is generally some kind of race battery. Interstate Batteries bumps up to a larger group size to get their fiberglass wrap battery.
To my knowledge, the rule wasn't about a heavy battery but about a light one to make weight in the 1.6 cars. Especially at 2275 there were alot of tiny batteries around then.
ALSO, I highly recommend everyone buys or makes one of these (AGAIN the rules specifically allow and highly recommend these):
http://www.advanced-...products_id=671
The steel plate on the bottom holds the shape of the fender cavity in most accidents. This would have prevented the mess I had at Miller. Most of the cars around here run these
My .02
Kyle
Short of making weight for a 1.6 rental, we run larger batteries. These guys really like to run cool suits, helmet blowers, and radiator fans when sitting in grid or in impound. The larger battery has kept the drama of the car barely starting or worse when it is time to go on grid, which ruins the race concentration of the typical renter, especially newbies (See I was 7th...
)
(BTW using the term fiberglass wrap as "AGM")