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

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.