While restoring my 1970 RT1, I found the piston had been filed 1 to1.5 mm at the auxiliary transfer ports. None of the cylinder ports have been modified.
Has anyone seen this before?

I replaced the piston with a new OEM part.
I'm aware of the piston mod in the service bulletin but this is different.
 

Old school modification.  Some of the books in the Tech Library probably describe it.  It's a way to modify port timing that can be reversed by installing new unmodified piston.  I did it to a few bikes when I was a kid.

Just to clarify, I never did that exact mod...  usually I just did the exhaust port and sometimes all the transfers. 

Never just a couple transfers....  but that's the beauty of the method, you can experiment and if it doesn't work out it's not hard to reverse the mod.

Does it run any different with the new piston?  Seems like a peculiar choice as far as porting goes.

I don't know how it will run.  I'm still restoring it.  It seemed to run okay before disassembly. 

Thanks Mark 

I've still got a factory experimental piston for a YZ80 that was sent to us to try at work in about 1982, maybe a little later. It's modified all the way around "Except the Exhaust Port". Never got to use it in the end but a think it was to try & make it more rideable & less peaky in the power. Had super thin rings too.

I remember making a similar mod. to my AT1 back in 1970. Ran great! 
alnarv

A good friend of mine is a former Team Suzuki MX wrench in the US and Europe back in the late 70s-80s. He told me they would scallop the pistons to change power characteristics from track to track. Back in those day's they didn't have room in the pit vans to stock that many different  cylinders. A box of pistons with different scallops was much easier to transport. I'd believe him, I've seen his old pictures of him and the team riders.

Found my pics on a post back in 2016. Was a YZ80J 47mm piston so was a long stroke motor to start with earlier being 49mm by memory. But must've wanted to see if they could get more torque out of it. By the time it arrived all our local ones had been bored oversize for various reasons so never got to find out if it helped.