Date code in components marking
The Matsuo date code is explained in more detail in their other datasheets, such as p. 2 of the Matsuo Tantalum Solid Electrolytic Capacitors datasheet.
The codes repeat every 4 years:
The 12 uppercase letters A through M (minus "I") represent the 12 months Jan through December for one of the years 1997, 2001, 2005, 2009, 2013, 2017, ...
The 12 uppercase letters N through Z (minus "O") represent the 12 months Jan through December for one of the years 1998, 2002, 2006, 2010, 2014, 2018, ...
The 12 lowercase letters a through m (minus "i") represent the 12 months Jan through December for one of the years 1999, 2003, 2007, 2011, 2015, 2019, ...
The 12 lowercase letters n through z (minus "o") represent the 12 months Jan through December for one of the years 2000, 2004, 2008, 2012, 2016, 2020, ...
The capacitor you have with date code "M" was apparently manufactured in December in one of the years 1997, 2001, 2005, 2009, or 2013. (Since you had it in your hands June 2016 we can rule out any later years).
The Jaro date code looks like it might be exactly the same as the Matsuo date code.
This coding standard is defined according to Japanese Industrial Standard JIS 5101, table 12 (as stated in the datasheet linked by davidcary) and in the Corresponding International Standards IEC 60384-1:2008 that refers, for the coding method, to IEC-60062, section 6.3.1 – Four-year cycle (small components, SM and THM only).
IEC-60062: Table 12 – One-character code – 4-year cycle
N.B.: The letters "O","o", "I" and "i" are omitted to avoid ambiguity.
For the same reason, IEC-60062 stated that:
If there is a possibility that a single lower-case letter could be read as an upper-case letter, for example, v for V, the lower-case letter could be marked with a cross bar above it.
Example (from the Original Question)
The capacitor Matsuo 553 with date code "M" was manufactured in December in one of the years 1997, 2001, 2005, 2009, or 2013. (I know the PCB came from '90, then the year is 1997).
The Jaro date code looks like it might be exactly the same as the Matsuo date code.
(Thanks to davidcary)