Local UFO Recurrency :
Pre-1948 sightings
This is a Local Recurrency frequency spectrum, but
with initial sightings limited to dates in or before
December 1947.
The peak at 5 years for return UFOs simply reflects the intervals between the great waves of 1947 and 1952, and between 1952 and 1957.
A surprise peak is centered at 578 months, 48 years and two months. This mostly results from 1995 sightings in the USA, and most of those came from the National UFO Reporting Center in Seattle, WA. While no real wave took place in 1995, it was the busiest year since 1973.
This effect vanishes when limited to sightings outside of the USA.