The part number is available in the object's Info dialog box; in a script,
it's available in the object's partNumber property.
Changing Part Numbers
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When an object's position relative to other objects within a domain
changes, its part number changes -- as when it moves closer to, or farther
away from, the front.
Lowest and Highest Part Number in a Domain
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The object farthest away from the front within its domain has a part number
of 1; the object closest to the front in its domain has a part number equal
to the total number of objects in that domain.
This article was adapted from the HyperCard 2.2 New Features stack.