FMP has a Thomas Hy Mason listed in the "Chelsea Pensioners' discharge documents 1760-1887" database. Page is headed "Scots Frontier Regiment of Guards". In the "Battalion" column it says 2900 2. I'm guessing that 2900 is his army number and 2 the number of the battalion? Rank is Private. Applicant is Regiment. Date of application 18th May 1875. Character Good. Cause of discharge - Miscellaneous (services no longer required.) Amount of service towards GC pay and pension 3 yrs 36 days. Date of discharge 25 May 1875.
Although this would suggest his service began in 1872 and so couldn't be your Thomas, I think it could be that they only counted service after his eighteenth birthday.
This is the link to the image which should be free to view today:
http://search.findmypast.co.uk/recor...hlights=%22%22