Blendin Replacement Jar Base Bottom Cap, Compatible with Oster Pro 1200 Blender

why does this bottom cap have the extending side prong? i thought mine was broken but it looks just like your photo

How can you push the prong in? Mine is extended out so much that it won’t fit into the blender.

How can i get a glass blender jar compatible with oster pro 1200? it's bottom is wider than most.

Which jar bottom and blade would be compatible with the Oster pro 1200?