The pics will show you that there is about 1/4" difference in these two grills. I found this out by mere accident when I was restoring my 1964 M151. At that time I had an A1 grill and hood. When I found the correct hood and installed it had a down ward slop to it. High in the rear & low in the front. So I try to readjust the grill but still getting the slop. So I quit betting my self up and went on. A friend came across some A1 grills and asked if I wanted them and of course I took them and one day when I was moving them into storage I notice one a bit odd and didn't match the rest. So I retackled the situation once again. I replaced the A1 grill and installed the odd one and the slop was gone. M151A1 grill from top to bottom measures 18.75 inches. The M151 grill measures at 19.00 inches. Also another way to tell is to take a #3 Philips screw driver i.e. a go / no-go tool. On the backside if it passes over the headlight bucket mounting tub you have a M151 Grill and if it don't pass over then you have an M151A1 Grill. The OD Green Grill is the M151 and the Grey Grill is the M151A1