This usually will tell it all. In most cases this part number should be a 660 or a 661. The M-21 was a 660 and it had a Drain Plug, the 661 was a M-22 and they had a drain plug. So therefore, this case is not a "Late Model" 1969 or Newer Muncie! Matter of fact it is probably a 5010 at best. Any other case has the Failure Prone 7/8ths inch Counter Gear Shaft!
