Sorry guys. I transposed two numbers when looking up the relation.
"data_pk" wasn't the right index.
=# select (page_header(get_raw_page('index_data_id', 'main', 3020854))).* ;
lsn | tli | flags | lower | upper | special | pagesize |
version | prune_xid
--------------+-----+-------+-------+-------+---------+----------+---------+-----------
CF0/2DD67BB8 | 5 | 0 | 792 | 5104 | 8176 | 8192 |
4 | 0
(1 row)