Built into the FirstRib "magic" initrd.gz is the option:
w_00modules : special option only - specifies 00modules arrangement as either lib or usrlib
This kernel command line parameter is rarely used but is a powerful option. This option parameter will allow a FirstRib based Kennel Linux to use either a usrmerge or a non-usrmerge huge kernel. Allowing the use of pre-usrmerge kernels that provide Overlayfs.
This means the KL can boot with 00module SFS's that have the /lib or use the /usr/lib format. The 01firmware SFS must of course match the format that the 00modules SFS is using in this use case.
the w_00modules option is another small useful parameter option that can make the difference when supplying a KLV for example with an older huge non-usrmerge kernel when working on "older" computer hardware.
A similar option is in more recent Puppy Linux's with the punionfs=overlay parameter