I am wondering about a melon..

Burpee Melon Early Silver Line Hybrid, from what I can find it was introduced in this country in 1965 through Burpee seed ...who knows how long it had been growing in Asia prior to that.

It is listed as an Heirloom and a Hybrid..some references say F1, others say nothing...so when is the seed from something like this considered stable enough to save pure seed from it?