Hungarian Buildings - Underprint?

We've all seen these stamps from Hungary of buildings. There were three issues of them in 1951, 1952 and 1953.
Removing a bunch stuck to some album pages, I found these two with a surprise on the back - a red postal horn.
The 3ft stamp on the left is from the 1951 set, while the stamp on the right is from 1953.



On the back are identical red postal horns.



I'm just curious about these. Scott's doesn't mention them. Were they stamped there for security? Are they simply lifted from whatever card or envelope the stamp originally traveled on?
I'm am really intrigued. Anyone know?

I am almost POSITIVE I've seen that same thing when I was sorting my stamps... Its buried somewhere in the Hungary pile now, but I am 99% sure I've seen it.

I don't recall the exact stamp it was on though...(or maybe even the exact country)

My guess they were added after,
they seem to be on the paper backing
Childs play stamp, although the posthorn is a query
I believe the posthorn is on the back of early Swiss is it?

https://goscf.com/t/8557