The individual segments, "bars" on each digit are not individually replaceable. Each 7-segment digit is a separate part. Original part number is H-P type 5082-7623.
If you replace just one digit, you'll find that the brightness and color will not quite match the other five. When they are manufactured, the factory will sort them by color and brightness, and include a suffix letter/numeral to the type number. The added marking allows you to "match" digits for appearance.
If you just want to fill in the missing digit, a single replacement is out there somewhere. Changing all six digits to make them all match sounds a bit over-the-top to me.
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