The difference you note is minor. The difference you note is a monogram. It may denote the mint or be just some other type of mark the minters used to regulate the coinage. There is no doubt that your coin is of Demetrios I of Baktria. Here is another example that is similar:
http://www.acsearch.info/record.html?id=75809
Try looking at
www.cngcoins.com or acsearch.info and you may find an exact match.
Matt