Since you called it a slasher, I'd dare to guess that you may be from Australia? England?
It could be that there are slashers built for those markets that are designed to operate at a higher RPM than those designed for US consumption, which is 540 RPM.
You should assure that you know the proper speed for that particular mower, because over revving may lead to overheating and ruination of the gear box.
John