Matt filed Sept 1918, the war was over by Nov 1918, and with his age it is very doubtful he was drafted or served. The WWI Draft Registration was a civilian one and having a card does not mean a man was drafted and served. It fact if one doesn't find a card for a male that should have filed then more investigation is needed, as those who volunteered and were already in the service did not file.
The 1930 census has column 30 for the question "Whether a veteran of US Military and Naval forces--Yes or No" and 31 "what war..." and Matt N Brost living in South Gate, Los Angeles County, CA answered "NO". So he was not in the any Military Service for a "pension". FYI: there was no "pension" paid to WWI Veterans. They were voted by the Congress in 1924 to receive a "bonus" payable in 1945. Google "Bonus Army" for more information. All service records and whether a military member received a pension in WWI and later years is not on the internet (yet). They are in a individual Veteran's service record at the NARA and one must be "next of Kin" to get copies.
Sorry I don't reply to individual email addresses on the boards.