My dad pulled out an old colt diamondback from the safe and was wanting to know if he could shoot .38+P out of it. Can the gun take that extra load? The only caliber stamp is .38 special CTG. and the cylinder doesn't look too heavy duty so I'm venturing to say it can't shoot +P, am I right?

It is not advisable, if you want to shoot +P ammo thru a .38 Special, do it though a gun rated for it. The .38 Special Diamond back isn't rated for it.

...one of the most beautiful revolvers ever made.

Please don’t beat it to pieces.

Thanks, will let him know. Apparently the only .38 ammo he's seen in stores is +P that's why he asked.

Colt rated the "D" steel frame models after 1973 for "up to" 3000 rounds of +P.
The post-1973 aluminum guns were good up to 1000 rounds.
After that amount the gun was to be sent in to Colt for inspection and possible frame replacement.

The Diamondback always had a beefier frame then the Detective Special and Cobra, so the Diamondback was really good for limited +P even prior to 1973.

Shooting a limited amount of today's +P ammo in a Colt small frame isn't going to destroy it, but it will wear a little faster.
Today's +P is lighter loaded then the original version that was a lot hotter.
Even in the 50's some people shot standard .38 Special ammo for practice and loaded hot loads for carry.
That still works well today.

You can find mild 38 wadcutters on line for your dad, and cowboy action ammo, which is even easier to find, is generally even milder.

A cylinder full of +Ps for the nightstand won’t hurt the Diamondback.

FWIW, here’s mine.

I would show kindness and mercy, and skip the +P rounds. I would make it a house gun. My preferred house guns are a .45, that I bought in 1980. It has gotten a lot of TLC.
And my other preferred gun is a S&W .40, it’s one of the early .40’s and built like a crowbar, strength wise. That’s my do it all gun, and I think it could handle +P+.