I immediately answered that it's a document kind code: "A" is used for the first publication, "B" for the second and "C" for the third. However, something was odd about their presence in espacenet. The USPTO only started applying kind codes to patent documents in 2001, so why did they appear in older patents? Apparently, espacenet is retrospectively applying them to patent numbers.
This might be confusing to some searchers because the "A" kind code is generally associated with published applications, not issued patents, which are usually marked "B" or "C". However, some countries only recently started publishing applications. For example, the US in 2001 and Canada in 1990. In these cases, the issued patent was the first publication, hence the "A". From Jan. 1, 2001 forward, US patents are marked "B". Canadian patents were marked "A" prior to Oct. 1, 1989 and "C" afterward.
