Arrow IV altimeter
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The FAA Pilot's Handbook of Aeronautical Knowledge (PHAK) contains this explanation...
"A crosshatched area appears on some altimeters when displaying an altitude below 10,000 feet MSL."
The complete PHAK is available for download here... https://www.faa.gov/regulations_policies/handbooks_manuals/aviation/phak/
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@retiredman93231 thank you