What’s the best symbol to use to mask unknown digits?

I'm working with groups of product identifiers and need to be able to represent when the last couple numbers of the identifier are unspecified.

For example, a product might have the following product IDs:

1000-1234-01
1000-1234-05
1000-1234-10
1000-1234-50

I would like to represent this group with just the first eight digits, and use a masking character to indicate that the IDs are actually 10-digits long but the user doesn't need to worry about the last two digits, for example using X characters:

1000-1234-XX

But I'm concerned the X character may not be universally recognized as a "placeholder" character. They might confused the X as part of the ID. (My users are in the US, but I can't assume their cultural background).

Other options I can think of:

  • Elipsis: 1000-1234-…
  • Underscore: 1000-1234-__
  • Times: 1000-1234-××
  • Whitespace: 1000-1234-
  • Question marks: 1000-1234-??
  • Asterisks: 1000-1234-**
  • Bullets: 1000-1234-••

The masking character is less ambiguous when users will recognize the format of the number, e.g., for credit card numbers XXXX-XXXX-XXXX-4000 or phone numbers (555) 123-XXXX, but is more critical when you want to avoid ambiguity.

What is the best character to use in this case?

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