Most likely a small male flower early on in flower.
Since the seeds are well matured, your plant must have been pollinated quite early in flowering.
Actually, this early pollination may have stopped further male flowers from forming due to hormones in the plant telling it that breeding is going on.
The seeds will be regular with a chance of being either male or female and most likely with a higher chance of being hermaphrodites themselves unfortunately.
They will also still carry a dominance for being autoflowers.