75mm x 465, Mk iii, M1920

Can someone tell me about the gun or ammo pictured, case 75mm x 465?

(Edited changing incorrect 490mm to correct 465mm)

There is a good article on these and similar cases and guns in IAA Journal 440 pages 34-36. Rick Seid wrote about “High Power 3-inch Ammunition for the U.S. Army.”

Mostly seacoast and AA guns, but pretty complicated so best to read the article.

Thank you for all the info you share on your many platforms!

Thx John, I read the article but didn’t find mention of this long 75mm round. Where’s Duguid these days?