There are numerous numerous variations of the mason's with a number in the Red Book ranging from # 2029 to 2068 ( 2, 8 & 9 ) all of which with the exception of rare embossing are 10 - 15 give or take. The O on the bottom is a valve mark left by the machine on which it was made. You did not specify if the lip was smooth or ground ( ground lips are a bit older ) and these were made by numerous different company's around 1910 to 30's give or take. Be careful as this looks like the start of a collector obsession which has led many to have whole rooms full of jars!