From page two of the paper:
"Low-period oscillators in Life, roughly p ≤ 15, can be found by playing with patterns by hand or
using brute force computer searches. In 1996, David Buckingham demonstrated [6] using his “Herschel
conduits” that one can construct oscillators with p ≥ 61 by sending signals around a closed track; the
cutoff for systematically constructing oscillators was later improved to p ≥ 43, by Mike Playle’s discovery
of the Snark [43]."