Sterling Silver Mark (reading correctly?)

American Silver Reading American Silver Hallmarks
1

Hi, I’m trying to figure out what sort of sterling silver pieces my mother has, and one mark is a bit confusing… on the bottom of a bowl, the marking appears to say:

Sterling 066

I assume that is just “990” printed upside down for some reason? Why else would it be marked as “sterling”? I’ve included a photo here… thanks for any help. Jonathan1959

IMG_0856
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2

This is the mark of Mueck-Carey Co (American Silver Marks M6 - Online Encyclopedia of Silver Marks, Hallmarks & Makers' Marks). The ID number may very well be 066 - 3 digits used in anticipation of reaching 100? Anyway why put the number upside down? Sterling is 925 and 990 silver would be too soft for purpose.

Phil