The 1386303 can be identified by the square silver logo on the right hand side, the non-removable cable and the extra key on the numpad. Most other versions of the model M keyboard have a dark Minus key spanning two rows on the numpad. Since this keyboard does not feature a PS/2 connector a converter is needed for use with a PC.
My 1386303 has a very early production date, a low serial number an like every Model M produced in the US up to May of 1986 a barcode on the label.
This 1386303 is probably one of the oldest model M keyboards in the world.