Installed to allow easy access and torquing of gear bolts. The added thickness of the retainers requires a longer bolt. I installed NAS6606-36 bolts, same grip slightly longer threaded end. These bolts are for Showplanes gear the stock Vans gear will require a different length bolt.
The retainers are sold by a guy on VAF but he is not easy to find. His email is firstname.lastname@example.org $40 a set as of this posting.