[Masked][Specific Chars][Masked][Specific Chars]

[undeath]

I don't understand what you mean. You're using the word "mask" in a weird way. And what does all that have to do with a wordlist?

xxx is a known fixed string (or two known fixed strings?) from what I understand and Y is a placeholder for small set of characters (but each Y may be a different character), is that correct?

So assuming Y can be 1 2 3 4 or 5, your candidates would look like

somefixedstring111somefixedstring111
somefixedstring111somefixedstring112
somefixedstring551somefixedstring232
somefixedstring551somefixedstring2324
somefixedstring551somefixedstring2222
somefixedstring234somefixedstring1111

Is this what you're looking for?