Folder filenode in tablespace location not indentifiable by oid2name

Hi all.

A tablespace is created successfully and so is a test table in it:
# CREATE TABLESPACE ts_name LOCATION '/path/to/tablespace/location';
# SET default_tablespace=ts_name;
# CREATE TABLE ts_test_table;

From the command line:
$ cd /path/to/tablespace/location
$ ls
PG_9.1_201105231
$ cd PG_9.1_201105231/
$ ls
11955
$ cd 11955/
$ ls
24683 24687 24689

All files 2468x correspond to objects identifiable by oid2name:
$ oid2name -d postgres -f 24683
From database "postgres":
Filenode Table Name
-------------------------
24683 ts_test_table

But these give nothing:
$ oid2name -d postgres -f 11955
$ oid2name -f 11955

Any ideas what it refers to?

thanks,
thalis k.