Not true at all, I have taken many of these off original cars and found there are 3 Variations, Upside down Left Bottom, Right side up Right Top and NO Delco Moraine Stamp at all. Just speaking for 1970 Chevelle though as I have had many of these.
Here is a photo of one I have for sale in the Classifieds that Helmut is buying to just throw in an example of the most typical BW Born With Unit

Here's one....

I actually don’t mind these repops...for the price they look decent and work well. Way better than the typical parts store booster that looks like it belongs on an ‘87 Buick Century.

The best restorer of original boosters has a comparison on his website.
Thank you, Steve Gregori:
http://brakeboosters.com/reprovorg.htm

I agree!! We donated the pristine original to a high quality concourse resto effort, and installed a repop - I want Ben to drive the wheels off this thing!! To be honest, I thought the repop look pretty good.

One thing to know is a little tip from Jed P here in PA, the receptacle on the face of the booster is too long - probably for multiple applications, and thus it effectively maintains constant brake pressure. This is a small bakelite type of item, and it must be ground flush to prevent the brakes from overheating....

Lynn , hi where did you get your original replated/ rebuilt ?

There was a Buick GS that had a severe brake failure with a repop . The halves of the booster turned and the booster came apart. The tabs were never locked into place to prevent it from twisting . Car was not damaged but his boxers were totaled ..

I've sent an original booster to Steve Gregori to have it redone and he's brilliant at restoring them if it's not heavily pitted. Sometimes you buy a donor at $200 and then send it to be rebuilt at say $350 and you can easily have $500-$600 into by the time it's back. Not practical for a driver car IMO, and so easy to use the repro at $114 which now includes the stamp. I just used one on this Nova I'm doing and as long as it works..I'll be a happy guy with it on there.

Agreed. I have my original that I'm going to have Steve re-plate at some point. For now, I bought one with the "correct" upside down stamp. Works for me for the time being...

They look better with no stamp at all IMO.