Does reaching base on a Def error count as an AB in the player's BA? Thanks

The answer to your question is "YES"

Food for thought.... today, the official scorebook recognizes a "sacrifice fly" NOT as an official at bat. In the 40's it was considered an official at bat..... Now you can understand why the hitters of that day (Joe D. & Ted Williams) are still marveled at even today. Think about it.... using todays official scoring what there real batting averages were.
Unreal!