If the second variant pops up, there is a pair of buttons that will switch between the two formats. If the first variant pops up, there is no set of buttons and I see no way to switch.
It seems to happen when I paste block into a file. Then I have to exit Rhino and then I can insert the block. It does not happen all the time.
I think that’s by design.
If you run Insert in a new file with no blocks defined in it, you get the top dialog.
If the file has blocks defined in it you get the second.
I thought it was sometimes happening when I had blocks defined. Here’s an example. You can see two blocks defined but it will not let me insert a block.