IS THIS A LARSON STORM DOOR HANDLE ?

Thanks for posting. I am afraid we can't provide another key for this exact lockset; it's impossible for us to know how the key should be cut, since this is unique to each lock cylinder. The best we can offer is a replacement for the entire lockset, complete with new keys: see our 40-058 or 40-177, depending on the style you prefer. I believe this lockset would work for your door, but see the dimensions below to verify. Otherwise your best option is to take the lock cylinder to a locksmith and see if they can cut a new key for you.

Alternate image for 40-058

Inside handle of 40-058