Here is some more "scoop" on these engines:
Another interesting cylinder block
feature was that there were a nu
mber of special experimental
engines (between 1353 and 1355) that were also
produced between July 9, 1928 and October 2,
1928. These special cylinder blocks did not incl
ude the A-6645 oil return pipe assembly, along
with the oil return pipe connections on the A-6015 cylinder block and the A-6520 valve chamber
2
cover. The addition of oil return holes were drilled on the floor of the valve chamber to allow the
oil to drain back into the oil pan along with other special features.