Here is a quote from the link below..
"Engine Block
This one is easy. Examine the serial number stamped on the engine (see serial number page for location). The number prefix will tell you the manufacturer. Willys engines have numbers that start with the letters "MB" (MB123456) whereas Ford engine numbers usually start with "GPW" (GPW123456)
If your serial number is not located as shown on the serial number page, then you probably have a post-war replacement or civilian engine block in your jeep."
http://www.rensjeep.com/MBvsGPW.htmlLooks like you have a Willys MB engine.