I work with Jerry Taylor, Bob Schnabel, and Dave Patterson at the University of Missouri. I am actually part of the research. Dr. Taylor works quite frequently with Dr. Beever. In fact we (Mizzou) did the 50K genotyping that lead to the hypotrichosis test in Herefords. And Dr. Beever is not the only person who identifies mutations responsible for genetic defects.
http://www.plosone.org/article/info:doi/10.1371/journal.pone.0015116
The 76 mutations cause a major change (damaging amino acid substitution, protein truncation, disrupted splice site, etc) to the sequence of proteins that are shown to be lethal in mouse knockouts.
http://en.wikipedia.org/wiki/Knockout_mouse So, these are damaging changes to proteins that are necessary for life. I called them putative because we have not yet validated them. In the second part of aim 1 of our grant
http://cris.nifa.usda.gov/cgi-bin/starfinder/10125/crisassist.txt we will genotype 10,000 heifers to identify the frequency of these putative lethal mutations. If we never see the mutation as a homozygous (the animal inherited two copies) then, it is a recessive lethal.
Furthermore, you do not need to observe a dead calf to confirm a lethal mutation. Most lethal mutations cause an abortion early in the pregnancy. Thus, the fetus has died long before 9 months.
Using your previous math, 150 bulls * 6.9 mutations per bull would give us 1,035 lethal mutations. I doubt we will see that many, because many of these may be shared among bulls.