Hopefully your stickers at the right rear of the tunnel are still legible.
In the 1st pic, look at the bottom line, or "Market Code".
If it says.... SS340-A1 it's a 1978
SS340-A2 it's a '79
SS340-A3 it's an '80
There were no 1981 340 Invaders....just the 440 in '81.
The 440's used the same code, except SS440-A1. etc....and the
'81 440 was SS440-A3A....(instead of SS440-A4....for some reason).
The 2nd pic shows the build date....on mine, the month is missing but you can see the year...78.
Anything built in the summer/fall of 1978 would be a 1979 model.