The Tomb of the Unknown Type?

We have encountered a pretty oddball situation involving an "unknown" type.

mydb=# select version();                                                version                                                  
----------------------------------------------------------------------------------------------------------PostgreSQL 7.4.2 on i686-pc-linux-gnu, compiled by GCC gcc (GCC) 3.2.3 20030502 (Red Hat Linux 3.2.3-24)
(1 row)

mydb=# \d redact_current24248       Table "public.redact_current24248"        Column          |   Type    | Modifiers 
-------------------------+-----------+-----------n_posted_transaction_id | integer   | n_year_u                | "unknown" | n_month_u               | "unknown" | n_breakdown_config_id   | integer   | n_amount                | numeric   | 

We'd like to turn those "unknown" values into plain integers (e.g. -
years and months); apparently it's not so simple...

mydb=# select n_year_u::integer, n_month_u::integer from redact_current24248 limit 10;
ERROR:  failed to find conversion function from "unknown" to integer

How this was generated was with Perl code where the prepared query
looks something like the following:

"CREATE TEMP TABLE $tableName AS ".
"SELECT a.id as n_posted_transaction_id, ".      "? as n_year_u, ".      "? as n_month_u, ".      "c.id as n_breakdown_config_id, ".       "calc_revenue( various_parameters ) as n_amount ".
"FROM ".   "transactions_posted a, ".   "items b, ".   "transaction_breakdown_config c; ";

I wasn't aware of there being an "unknown" type, and it's rather
bizarre that this is happening.

I imagine that specifying
"SELECT a.id as n_posted_transaction_id, ".      "?::integer as n_year_u, ".      "?::integer as n_month_u, ".

would likely clear this up, but where "unknown" came from is something
of a mystery.  The source types shouldn't be any mystery.
-- 
"cbbrowne","@","cbbrowne.com"
http://cbbrowne.com/info/x.html
str->str_pok |= SP_FBM;                     /* deep magic */
s = (unsigned char*)(str->str_ptr);         /* deeper magic */
-- Larry Wall in util.c from the perl source code