On OEM engines for service in the field, this hole was where a tube from the crankcase breather connected back to the carb air filter box. Some engines sold by 600 used to come with a platic plug in this hole, some did not. Engines from 600 now come either with the hole open or with duck tape covering the hole. Pursuant to my conversation with Scott Rhinehardt at INEX yesterday, competitors may now run the engines with the plug or tape, or without any plug or tape. It is optional for the competitor to choose.