They cross the 7 (to distinguish it from a 1) in many places; actually, a lot of people in New York do it, probably because of the influence of so many 7-crossing foreigners. I myself cross my 7’s because I travel to places where they do it, and it makes one’s numbers clearer to them. Reading foreign numbers can be difficult!
That said, someone I know here (in Florida) got an e-mail death-threat from somebody who called himself “Ali the Assassin” and closed his threat-letter with “ALLAH IS MERCIFUL.” She thinks it was just spam, and not personal, because she was supposed to get in touch with an e-mail address in the UK (presumably to send money to buy off the “assassin”). Needless to say, she sent it to the FBI. But there are no doubt other people who received it and were terrified by it - anybody here get one or know anybody who did?
A handwriting analyst could probably tell a lot about the characteristics of the person who sent them.
Here's a WW2 POW record that will make clear how some foreigners got in the habit of crossing sevens:
They write their ones with long serifs that make them look like sevens, except then they cross the sevens, resolving the ambiguity. For example, it is clear that Waffen SS Pvt. Grass was born on October 16 Okt 1927, not 76 Okt 1927, that he was 18 years old, not 78, and that he was 171 cm tall, not 777.