MN_Moose,

With the muffler removed, look through the exhaust port at the piston. If it is a one piece unit, from bottom to top, it is an ABC (or ABN; same technique), If there is a ring there, you will see it.

The MDS site is:

http://www.mdsengines.com/

I don't see a .61 there, just a .68 and a .58, which are ABC.

You can contact Horizon Hobbies, the importer to the USA, through that web page.

Sincerely,