I have made a small discovery this morning about the 1876 United States (Philadelphia) International Exhibition Tickets.
Evidently the purpose of the Red Fifty Cents overprints seen on either blue or red serial numbered tickets was put on the tickets because those without the overprint have a small Fifty Cents already on them above the
ADMIT THE BEARER text on the front.
The discovery has to do with the
"Pink Back" or
"Red Back" tickets with the red serial number on it.
These are different than the "White Backs" because the white backs do have the small Fifty Cents printed on them while the "Red Back" does not.
There may be other variations to be found out there after seeing this small but definite difference.
But now, my 4 ticket set has increased to 6 if I count what I think is a green fake or counterfeit.
I have found the green in grey ink with the same serial number on it on the internet. I have a green example.
Here's my new collage.

There are FIVE tickets I have found to make my set of SIX for this type ticket.
Top Left has a
GREEN serial number 89048 with a Green
FIFTY CENTS overprint in green, and the front ink color is entirely green. The back is printed with black ink.
I believe to be a high quality fake, reprint or counterfeit. I have seen another ticket similar with same serial number
89048 as well in all dark gray ink.
Top Middle has a
BLUE 95331 serial number with a
FIFTY CENTS overprint in red
Top Right has a
BLUE serial number
69359 with small FIFTY CENTS above the ADMIT THE BEARER text on the front.
Bottom Left has a
RED serial number
31138 with a
FIFTY CENTS overprint in red
Bottom Middle (Red Back) has a
RED serial number
74956 without FIFTY CENTS overprint and
without FIFTY CENTS above the ADMIT THE BEARER text on the front.
Bottom Right has a
RED serial number
97078 without FIFTY CENTS overprint
with small FIFTY CENTS above the ADMIT THE BEARER text on the front.