Mac's site is great. But there are no casting dates that anyone has been able to decode. Closest you'll get are the various part prefix numbers cast on some heads.

Out of all the numbers cast on your bellhousing, only 59 has significance.

Only dated flathead is the 8BA/8RT/8CM series. They weren't cast dates, but were stamped on the passenger rear corner of the intake flat as an alpha-numeric string.

Congrats on the lucky find! Enjoy it.........