This site gives lists of various serial numbers for Conn wind instruments.
If it was made in 1987 or later:
From 1987 on, the two-digit prefix number plus 50 will give you the year of manufacture on all Conn products... Data for the years 1967 to March 1974 is rather sketchy, for it was around that period of time that the Conn Brasswinds were being manufactured in Abilene, TX. Instruments made during that period are not likely to be their best examples, as the workers were still relatively new at the manufacturing process.
This theory, unfortunately, does not account for the J.
If it was made between March 1974 and 1986:
In March of 1974 an alpha-numeric system was established, where the first character (letter) of a serial number indicated the decade; "G" for the 1970's, "H" for the 1980's, etc. The second character, also a letter, indicates the month of the year; "A" for January, "B" for February, "C" for March, "D" for April, "E" for May, "F" for June, "G" for July, "H" for August, "I" for September, "J" for October, "K" for November, and "L" for December.The third character (a number) indicates the number of year within the decade (indicated earlier by the first letter). The fourth character (number) indicates the specific type of instrument: "1" = Cornet, "2" = Trumpet, "3" = Alto Horn, "4" = French Horn, "5" = Mellophone, "6" = Valve Trombone, "7" = Slide Trombone, "8" = Baritone Horn & Euphonium, "9" = Tuba, and "10" = Sousaphone. The remaining four numbers of the serial number indicate the production number of the instrument on a monthly basis.
x -- either G for the 1970s or H for the 1980s.
J -- made in October
4 -- made in 1974 or 1984
1 -- first digit of "10" for sousaphone
x -- should be 0. second digit of "10" for sousaphone
x -- production number digit one
x -- production number digit two
x -- production number digit three
x -- production number digit four
This theory requires that there be six more letters, hidden under the paint.
This page about Jupiter instruments also gives serial numbers:
... The age of Jupiter instruments may be determined by adding one to the first digit of the serial number. The resulting number then would represent the year after 1984 that the instrument was manufactured. For example, if the serial number began with a number "4" you would then add "1" to this number and know that the instrument was manufactured in 1985. If the first number was a "0", would indicate that the instrument was manufactured in 1991.
The page also states all instruments made 1990 and later have a suffix of J.
Thus, if it is a Jupiter sousaphone, it would be a from 1995.
However, here there are also problems. This theory does not account for what the "1" means.
These are the only possible manufacturers that I have found. Conn is an American company, Jupiter is Taiwanese. It is likely your sousaphone was French (your profile says you're from France). Three characters isn't much to go on. If you find anything else on it, update me.
I hope it helps...