You could use a custom format of:
"E33213"@"OP5"

It'll be up to you to type exactly two characters.

And the value of the cell will not include those special formatting characters.

JG wrote:

I need to enter over 200 serial numbers into Excel.

They all start with the same 6 characters and all end in the same 3 like so:
E33213**0P5

Can I format the cells so that I only have to input the 2 middle characters
and Excel will auto fill the other information?

The 2 characters I need to enter are letters, not numbers.

I tried the following without any luck:
E33213??0P5
E33213##0P5

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Dave Peterson