They updated the HS40's in the 1973 calendar year to the deeper square top shroud and steel flywheel like the HS50's that were introduced in 1972, They eventually changed the entire small frame line to that steel flywheel design. I think along with a little more rotational weight it they also move a little more air.....well yours wont with that blade set :laugh:
Good thing to check on those is to grab or give a light tap to each magnet on the inside and check that they are attached well, They are pushing 40 years old now, I have found them come loose pretty easy and will ruin your day if they do. If any are loose they can be reglued ( I Like to use JB weld), just make sure they go in the same spot they came from including its polarity with the magnet next to it.
The actual lighting coil/ignition setup will be the same, just the flywheels are different