411A
"Spring lock is fine, except, it is not then firmly locked down."
Incorrect. Spring lock down has the "yale" tongue engaged exactly the same as positive down lock.
stevef is also correct on the hydraulic component (hydraulic lock maintains up).
411A you do not know the DC3, I do. I also solved the single circuit down lock indication problem immediately on being exposed to it(during my DC3 type rating course in 1961). I proved my solution as soon as I flew it. NZNAC had been operating DC3 for about 15 years but had not solved the problem previously. The solution is trivially simple.