China bluetooth modules and RS232c with RTS/CTS
Rx and Tx are 5V TTL logic compatible.
UART_TXD and RXD uses 3.3V CMOS logic compatible.
TXD and RXD are low swing RS232 bipolar compatible ( lower since doubled from 3.3V Vcc rather than 5V Vcc. so ~ +/- 4V swing)


You need to ensure compatiblity with CTS/RTS 3.3V CMOS and ensure polarity is correct for CTS/RTS TTL.

TTL threshold is 1.2V
HCMOS threhold is is Vcc/2 or 1.6V so close enough and should be able to drive 1 TTL load.
CMOS input protection over Vcc is all that is needed from 5V TTL. (series 1K and Shottky diode clamp to 3.3V)