William,
Not so early I think you will find.
B20 transaxles basically started at number B20 0.
Our 4th Series car - chassis B20 3124 - has its original transaxle B20 1150 so this transaxle which is B20 1004 - will also most likely be from a 4th series car. However, like the engine numbers the original transaxle numbers jump around all over the place with very few being even in sequence. This is presumably because of rejections when bench testing and the inevitable delay in rectification.
What is the number of the transaxle fitted to your project car?
Chris