default or selected kernelversion does not have config value CONFIG_R3964.
Result is shown for kernelversion 6.8.2

Siemens R3964 line discipline

modulename: n_r3964.ko

configname: CONFIG_R3964

Linux Kernel Configuration
└─>Device Drivers
└─>Character devices
└─>Siemens R3964 line discipline
In linux kernel since version 2.6.20 (release Date: 2007-02-04)  
This driver allows synchronous communication with devices using the
Siemens R3964 packet protocol. Unless you are dealing with special
hardware like PLCs, you are unlikely to need this.

To compile this driver as a module, choose M here: the
module will be called n_r3964.

If unsure, say N.

source code: