"Cannon" as plural

Strange as it seems, cannon does appear to have once been a mass noun, like rain or infantry. Instead of saying rains, one says drops of rain. Similarly, instead of saying cannons, it appears that one either said cannon or pieces of cannon. Consider the google Ngram below:

Here, the curve for two cannon is higher than it should be, because of constructions like two cannon balls.

Tsuyoshi is right about cannons being the plural in the 1500's; Google Ngrams doesn't have adequate data before early the 1700's, but we can check Shakespeare, who uses cannon as a regular count noun. So the plural has gone from cannons, to pieces of cannon, to cannon, and back to cannons.

I do not know English well enough to say anything about the question from my personal experience. However, a quick look at dictionaries suggests that it may not be the case that the plural form of “cannon” evolved from “cannon” to “cannons.”

• The Oxford English Dictionary (OED) (cannon, n.¹ 2b; the linked page requires subscription) lists the use of “cannon” as a collective noun and as plural, but it does not state that this use is obsolete. The Merriam-Webster Dictionary also lists both “cannons” and “cannon” as the plural forms of the noun “cannon.” (I do not personally know whether the construct such as “32 cannon” is correct in the modern English.)
• The first quotation of “cannons” cited in OED is in 1525, which is earlier than the first quotation of “cannon” as a collective noun or plural in 1596.