Here are some pics and a drawing. The aluminum piece has a black plastic piece that sits inside its curve. It is bolted inside bracket to rail like drawing. The black curved foot on end of bimini leg gets interlocked when you align slots with cross pin.
Glad I could help. Just remember that you need to install parts in proper direction (forward or rearward) so it cannot unlock when bimini is being used.(wind could lift tops) You actually swing bimini beyond normal range to align slots. Very clever and secure design that still allows quick removal of frame if needed without tools. (For front bimini, you fold legs well towards rear (stern) to align slot for install/removal and rear bimini just opposite.)