Are there differences between USART modules in the MSP430x337 
and the MSP430x337A devices?

The URXE and UTXE module enables for SPI mode on the MSP430x337 
are combined into one single module enable USPIE on the MSP430x337A.  
Another important difference is that in the `337A the TXEPT bit 
in the UTCTL register should not be reset by user software. It 
is recommended to use a bis.b instruction instead of using a mov.b 
instruction to program the required selection bits in the UTCTL 
register to avoid unintentional resetting of the TXEPT bit. 
This incompatibility is due to bug fixes done in the `A` version.