Hello Jon,
I hope I have a "no nonsense" answer! Your pen most likely began life as a ballpoint; the spring is there to put pressure on the back of the ballpoint refill. (I have lots of NoNonsense BP-to-FP conversions - some came to me that way, and I converted others.) You will need to remove the spring in order to use a converter.
For me, the most reliable method is the chopstick trick, detailed in this
thread. It hasn't let me down yet, and I've used it successfully with several different BP models, including Sheaffer Targas and Cross Solos.
By the way, your pen is a "Vintage" model - marbled finish and gold-plated trim, including a disc set into the top of the cap. A very attractive combination. If I remember correctly, the Vintage series included marbled red, green and blue as well as solid black.
And I heartily agree: the NoNonsense pen should have remained Sheaffer's entry-level offering in perpetuity. The current Calligraphy pen still shows traces of NoNonsense DNA in its design, but it's a far cry from the original. You may want to try using your pen as an eyedropper (with a bit of silicone grease on the threads); the NoNonsense lends itself well to that application.
Enjoy your NoNonsense! (Be advised that they can be habit-forming.)
Cheers,
Jon
PS A quick edit to acknowledge that Andy has beaten me to the punch!