(This story is part of the Weekend Brief edition of the Evening Brief newsletter. To sign up for CNBC's Evening Brief, click here.) The idea that we are late in the economic and financial market cycle is one that even most Wall Street bulls won't dispute. After all, when the economic expansion surpasses a decade to become the longest ever and the S&P 500 has delivered a compounded return of nearly 18% a year since March 2009, how can the cycle not be considered pretty mature? Yet it's not quite that simple. Huge parts of the economy have run out of sync, at separate speeds. Some indicators have a decidedly "good as it gets" look, others retain a midcycle profile — and a few even resemble early parts of a recovery than the end. Friday's unexpectedly strong November job gain above 200,000 reflects this debate, suggesting we are not at "full employment" even this deep into an expansion. And the market itself has stalled and retrenched several times along the way, keeping risk appetites tethered and purging or preventing excesses. In the "late-cycle" category we find several broad, trending data readings: Unemployment rate and jobless claims at a 50-year low; consumer confidence hit a cycle peak and has flattened out; and the broad index of leading economic indicators has slipped from very high levels. Auto sales peaked a few years ago. Corporate debt levels are near extremes, profit margins have retreated from historic highs, and equity valuations are certainly full and in line with the latter phases of prior bull markets. But corporate-credit conditions are sturdy, and households have simply not loaded up on debt this cycle, in a long period of enforced and then voluntary sobriety after the massive credit boom and bust that culminated in 2008. This leaves consumers in good shape. And the housing market, a drag on growth for years after the crash, has now perked up and is feeding off supply-demand dynamics that are more typical of an early cycle environment.

What about the yield curve?

The summertime inversion of the Treasury yield curve — in which longer-term bond yields slip below short-term rates after the Federal Reserve has been tightening policy for a while — crystallized the debate on the cycle's effective age. Such an inversion, in the past, has started the countdown to a recession — but sometimes with a lag as long as two years. This indicator has been translated into a recession-probability gauge one year ahead by the New York Fed. Source: New York Fed It has turned lower since late summer as the yield curve has returned to its "normal" shape, but only in the 1960s has it ever climbed above 30% and fallen back to tame levels well ahead of any recession. Have there even been enough cycles for this pattern to qualify as a statistically reliable "rule?" Do the extremely low absolute level of rates now (similar to the '60s) change the interpretation? Was the inversion too shallow and short-lived to serve as a proper signal? Whatever the answers, Jason Hunter, technical strategist at JP Morgan, notes that stocks have tended to have some of their strongest runs after an inversion, late in a cycle. "The longer-term bull cycles persisted for nearly two years after the initial [Treasury] curve inversion during the past three business cycles, with the majority of the late-cycle rally acceleration phases unfolding within the year after curve inversion." The S&P on average has gained more than 20% over less than two years in the past four episodes before peaking. One way to view the summer tumult is as the third severe "growth scare" of this expansion, following those of 2011-12 and 2015-16. Both brought with them nasty 15-20% equity downturns, new lows in Treasury yields and forced central banks to become more accommodative. The Fed has referred to its shift from rate hiking last year to three cuts this year as a "midcycle adjustment," which would leave it on hold for now and summons happy memories of prior such Fed-enabled "soft landings."

'Still upside' for stocks