If they appear in inverse video in less, they are probably ASCII SI characters (decimal 15, octal 017).
You can verify this with od
$ od -ta yourfile |head -n 1
0000000 si sp 3 6 0 0 si sp 3 6 0 3 si sp 0 si
Here I can see that the test file I made has space, space, "3600", si, space, "3603", ...
If you can verify that the characters in your file are also ASCII SI characters, you can use this command to remove them:
sed 's/\o017//g' yourfile > yourfile.new
Note that the \o017 is: backslash, lower case letter o, zero, one seven.