May not be your case, but this is happening often when you swap engines with RW Tools and leave checked the box:
Don't change numbers when swapping stocks. If you are swapping engines leave that box unchecked to avoid problems.
To fix the missing number, go back to RW Tools and swap that engine for the same engine, but now leaving unchecked that box.
Doc.
Edited later: Just checked and no problem with GP-9 GN, it looks like having just one (fixed) number: 661.
2012-06-23_00001.jpg
See more here:
Trying to swap engines
viewtopic.php?f=29&t=5299
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Who doesn't have dog, hunts with cat.