The “in memory undo” buffer is actually a buffer for the redo that describes the changes to the undo.
If you look at x$kcrfstrand you can see some information about redo threads (buffers), both public and private. In the example below I have 2 public threads and 10 private threads, but four of the private threads have been dynamically disabled so only 6 of the private threads are showing a buffer address.
select indx, ptr_kcrf_pvt_strand, from x$kcrfstrand; INDX PTR_KCRF STRAND_SIZE_KCRFA---------- -------- ----------------- 0 00 3494400 public 1 00 3494400 2 20F27C80 66560 private 3 20F38080 66560 4 20F48480 66560 5 20F58880 66560 6 20F68C80 66560 7 20F79080 66560 8 00 66560 dynamically disabled 9 00 66560 10 00 66560 11 00 66560