Whose birthplace did Post 148 in Virginia meet early in its history?

A: George Washington's. According to the Colonial Beach post's page on the Centennial Celebration website, the post was chartered in 1932 but didn't get a building of its own until 1950. Until then, members met in numerous hotels and/or restaurants in and around Colonial Beach. One of these was the Old Log House Tea Room at the George Washington Birthplace National Monument, a few miles outside of town.

Coincidentally, the town itself was first founded around 1650 by the great-great-grandfather of fellow president James Monroe, and known as Monrovia.