I don't necessarily believe it has slid back. About the ONLY way that can happen is for the front flange of the cam to wear excessively and/or the front of the block to be worn off (the thrust face for the cam). If never seen either happen, though I have seen a case where the cam bolts were too long and ground into the thrust surface a bit.
Did you happen to use repop cam bolts or a repop cam-bolt-locking plate (which are sometimes thinner).
Also, if anything like the above happened - enough for the cam to move, then there would be some obvious metal in the oil pan - a LOT. I'd check that out.
Also, I noticed the cam lobe in the picture appears in about the right place on the lifter.
Probably chasing a ghost at this point in time.
