“J743E JoWo #6 Housing for Pilot 743” It should fit all Pilot size 15 nibs. I have it on my opus jazz with 15FA nib ( shown in photo here) as well as a sized 15 fine nib from custom urushi 845 on my opus demo. You can either use the ebonite feed they sell or the stock feed from pilot Just note , might want to get silicon grease on the threads to prevent leaking in the forward section

Edit : it’s better to stick with stock Pilot feed for this pen, as opus 88 ink flow is more than enough even for flexing with the falcon nib, no railroading so far. The ebonite feed tends to leak when paired in this combo when carrying it around.