Brian McManus

When their third album Silverbird came out later in 1973, the bandmembers in Silverbird are listed (mostly without surnames) as Mark (acoustic, electric, lead, and bass guitar), Reuben (acoustic and bass guitar), Gilbert (piano, Rhodes and organ), and Perry (flute and drums), plus Brian McManus (banjo and guitar).  The producer though is listed as Reuben Ortiz. 

(August 2013)