Here are a couple of links that tell the difference and offer everything you ever wanted to know about these arms:
http://www.vinylengine.com/a-guide-rega-tonearms.shtml
http://www.tonearm.co.uk/faq-tonearms-frequently-asked-questions.htm
http://www.vinylengine.com/a-guide-rega-tonearms.shtml
http://www.tonearm.co.uk/faq-tonearms-frequently-asked-questions.htm