Eric Russell in North Carolina, USA, offers this tip.
"I changed it [1500 fender] to a 1600. I made the backing piece/tapping plates for the 1600 style and fit that to the 1500 fender".

Inside 1500 front wing --- Inside 1600 front wing
Outside 1500 front wing --- Outside 1600 front wing

On July 12, 2024, Eric Omite in Wilson, NY USA wrote:
"I found that the 1500 parking lamp covers the 1600 holes. The 1500 lamp base I believe has both the 3 hole 1500 and 2 hole 1600, so mounts in the 1600 fine. See pictures below. So if you don't want to do any welding and hole cutting you can use the 1500 lamp base directly". The base covered the holes nicely, so I didn’t have to re-drill the holes. I was pleased that it fit so wel".

Note: This appears to show the 1500 lamp base installed upside down in tyhe 1600 fender.