Tengu, this was the first batch of P-51s to enter service with the USAAF. These had no letter after the P-51, hence they were just known as P-51s. The first batch was sent to England and were known as Mustang Is and were armed with four 50 caliber Brownings and four 30 caliber Brownings. Two 50s under the chin, which had a bulged faring, and one in each wing and two 30s in each wing. The Apache had six 50 caliber, two under the chin, with no farings, and two in each wing. Another easy way to tell an Apache is the landing lights, both of which are mounted in a double fixture on the left wing. Hope this helps. And if I have goofed here, someone please correct me.