What meaning do the question marks have?
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I traced the photo to this place in Okazaki (Aichi prefecture):
On their web page, they claim to offer the cheapest gas based on a crowd-sourced survey of gas prices elsewhere in Japan. The numbers shown on the signs are based on these survey results:
Further down the page, it mentions that the prices are rounded up to the nearest whole yen, which is why there are question marks displayed instead of numbers:
As you can see, the price at the bottom of the sign is for 18 litres of kerosene (灯油). This number has no decimal point, so the last digit hasn't been replaced.
As Dave suspected, these question marks have very little to do with 日本語 at any level. But I hope someone finds this useful and/or interesting :-)