I just went to the Web site for the Marx-Engels Archive and they have a
search engine for all their Marx texts. Since I have said to people for
years that Marx himself never used the phrase, "labor theory of value", I
did the search and only once did that phrase come back--from Lenin's
article "Three Sources and Three Components of Marxism". When I changed
the search to "law of value", 19 matches come up.
The significance of the issue is the extent to which Marx broke from the
classical economists in his whole conceptual framework. I hope the search
engine is reliable.
Note that using the Marx-Engels Archive for phrase searching may be a
useful thing for us to get into the habit of using.
Web Site for Research in Political Economy where you can click on for the
Archive (which can be accessed by LYNX, if you don't have a browzer):