Short/Split spacebar – new plate needed?

I purchased an ANSI hhkb f77 (split right shift + split backspace), and would like to modify its layout to use the short spacebar with 4 keys to the right.

A separate Q&A states: “You can split the spacebar on all the keyboards yourself – just order the smaller space bar ($4 on the extra keys product page), one extra barrel, and one extra flipper (as well as whatever 1.25U size key you want to use – check the extra keys product page)”.  Meanwhile the extra keys page mostly just explains how to order the short spacebar, but not how to perform the installation.

After looking at images of the disassembled board I do not understand how to add the extra barrel for this key.  Even if the extra pad exists on the pcb, do I need a different plate that has a fourth hole to the right of the spacebar?  It looks to me like there are 4 holes on the left (for ISO layouts) but only 3 on the right…  What am I missing?

Thanks!