GOOD NEWS
You're right, it may be kind of rare. I can tell you why it's that light blue, Google "Richard Petty Dynamark"..... It's probably worth $150, but bear in mind any kind of chassis parts are almost impossible to find.
BAD NEWS
At one time in the 70's, Dynamark was owned by AMF. If you're not familiar with AMF, they were the folks that nearly drove Harley Davidson into the ground (I mean even worse than now). Ask any Biker what they think of a mid-70's AMF Harley and be prepared for a string of bad words that would make a sailor blush.
Dynamarks are rare because they built a cheap, POC mower, and nobody bought them. The only thing AMF ever built with the Harley Davidson logo on it that is worth collecting were their 3-wheeled golf carts. There's kind of a cult following on them and they are worth collecting......