For those not familiar, clubhouse signatures were as they sound, signed by someone in the dugout on behalf of the players. Back then there was no malicious intent.
The Dodgers of that era were known to often utilize clubhouse signatures, and as mentioned above it was not uncommon for Jackie, Campanella and Erskine to be clubhouses. One indication that this is the case is when they are all grouped on the same panel of the ball.
How pervasive were clubhouse signatures? SCP auctions just auctioned off the Walt Alston collection including this 1956 team signed baseball from Alston's personal collection. And yet (amongst others) PSA deemed the Walt Alston signature on the ball a clubhouse signature
http://catalog.scpauctions.com/bids/...e?itemid=30392