This is former US Marines Tracker Bu136421, which still has '6421' visible on the fin and 'Marines' on the wings. It was registered to the California Department of Forestry (CDF) as N434DF on 23 Jan 1974, but obviously it was never taken in service and the registration was never applied to the aircraft. Instead it ended up as a spares aircraft for the CDF at Sequoia Field, where it remained until it was moved to the Castle Air Museum on 14 Nov 2012. On 1 Feb 2000 the same registration N434DF was issued to Tracker c/n 335C.