if you look at the korean war pix of cannon armed corsairs you can tell they are F4U-4b or F4U-4c (AFAIK the b model was intended for the brits, but they cannceled the order as WWII was winding down & they didn't want to pay for them, so the USMC got the -4b. the -4c model was ordered by USN &/or the USMC, but i dont think there is any actual difference between the two types) rather than F4U-1c by the different type of blast tube. the -1c has a long and narower blast tube than -4b or -4c. also the -4 carried M3 20mm cannon, which is a bit more powerful than the M2 cannon on the -1c