Multiple Entry question from Nati

Hello, we are a husband and wife who are US citizens. We traveled to Beijing, Shanghai, Xian and other cities in China this past September, 2014. Therefore, we already have Chinese visas which will expire on July 2, 2015. They indicate in "Entries" the letter "M". Does this allow us multiple entry into China?

We plan to visit Guangzhou in December, 2014. However, our stay will be less than 72 hours there. Are we ok with the M entries one that we have already obtained? Or do we even need another Chinese one since our stay in Guangzhou is less than 72 hours.

We are flying round trip to Guangzhou from Manila, Philippines, but our residence that we will return (our onward ticket) to after our stay in Manila is in Honolulu, Hawaii, USA.

Thanks.