I currently read chapter 8 Beta from Bali, Engle and Murray's book Empirical Asset Pricing: The Cross Section of Stock Returns and do not understand their estimation on the five-year Beta-factor (denoted as $\beta^{5Y}$) using monthly data. On p. 124 they write:
We also calculate market beta using monthly excess return observations over the past one, two, three, and five years, requiring 10, 20, 24, and 24 valid monthly excess return observations, respectively. Our choice to require a maximum of 24 monthly data points to calculate beta, even for the five-year measure, follows common practice when using monthly data to estimate beta.
How would $\beta^{5Y}$ with 24 data points differ from $\beta^{3Y}$ also using a maximum of 24 monthly data-points?
The only interpretation would be, that $\beta^{5Y}$ requires a data-history of 5 years prior to the estimation date of $\beta$, but only using the latest 24 available monthly data for estimation. However, on table 8.1 they report on a CRSP-sample from June 1963 - November 2012 an average observation of 3,958 estimations of $\beta^{3Y}$ and 3,992 average observations of $\beta^{5Y}$ per year. Assuming that $\beta^{5Y}$ requires a longer observation history prior to the date of estimation, one would assume less observations of $\beta^{5Y}$ than of $\beta^{3Y}$.
Additionally, does one know a financial paper following this common practice on estimating $\beta^{5Y}$ with requiring a maximum of 24 prior data observations?
[1] Bali, Turan G., Robert F. Engle, and Scott Murray. Empirical asset pricing: the cross section of stock returns. John Wiley & Sons, 2016.