  In this paper, which is the third installment of the author's trilogy on
margin loan pricing, we analyze $1,367$ monthly observations of the U.S. broker
call money rate, which is the interest rate at which stock brokers can borrow
to fund their margin loans to retail clients. We describe the basic features
and mean-reverting behavior of this series and juxtapose the
empirically-derived laws of motion with the author's prior theories of margin
loan pricing (Garivaltis 2019a-b). This allows us to derive stochastic
differential equations that govern the evolution of the margin loan interest
rate and the leverage ratios of sophisticated brokerage clients (namely,
continuous time Kelly gamblers). Finally, we apply Merton's (1974) arbitrage
theory of corporate liability pricing to study theoretical constraints on the
risk premia that could be generated in the market for call money. Apparently,
if there is no arbitrage in the U.S. financial markets, the implication is that
the total volume of call loans must constitute north of $70\%$ of the value of
all leveraged portfolios.
