Oops, I'm such an idiot. For reasons I don't really agree with,

all devices are by default known by their 'cooked' name, so
my change was wrong. I thought it was a hangover from old 'block
tape device' support which hasn't worked (if it ever did) since
v6/PWB.

So, the default tape name is now the same as Linux. Far out, man....

sys/sys/mtio.h

@@ -225,7 +225,7 @@ union mterrstat {
 #define	MTIOCGETEOTMODEL	_IOR('m', 8, u_int32_t)
 
 #ifndef _KERNEL
-#define	DEFTAPE	"/dev/nrsa0"
+#define	DEFTAPE	"/dev/nsa0"
 #endif
 
 #ifdef	_KERNEL

usr.bin/mt/mt.1

@@ -224,7 +224,7 @@ If a tape name is not specified, and the environment variable
 does not exist;
 .Nm
 uses the device
-.Pa /dev/nrsa0 .
+.Pa /dev/nsa0 .
 .Pp
 .Nm \&Mt
 returns a 0 exit status when the operation(s) were successful,