It appears some Arduino versions implement in-band, software flow control (XOFF decimal 19 and XON decimal 17). This means you can't send arbitrary binary payload data over the line.

See also https://forum.gigatron.io/viewtopic.php?p=228#p228

One direction for a solution is to disable this feature. That sounds a lot better than going to a 7-bit encoding. [This is a bit ironic, because sendGt1.py only exists because flow control was missing on the Arduino Uno in the first place.]

Another idea is to send uncompiled .h files... They are text and not difficult to convert on the receiving end.

[Edit to future readers: After LOTS of debugging, the issue is NOT related at all to control bytes or XON/XOFF flow control, but concerns with the length of the data messages ]