My Scott Regular and Air Plate Block Album has a page for the large plate block of 20 of #1597, dated 1978. If your page set does not, contact Amos Publishing and ask them for a replacement for the missing page.
Find your page showing the 1975-81 American Definitives. There will be spaces for 5 plate blocks - 1591 thru 1594 (9¢ - 12¢) plus 1599 (16¢).
The next page will have a space for a PB of 12 of #1596 (13¢).
The next page is the one you want. Yes, Scott did format the page for the PB of 20, but did not allow enough frame line clearance for the PBs when Scott mounts are used. That is a common problem with EVERY Scott album page - the frames fit the stamps only if you hinge them.
Note that I add the visible Scott numbers to ALL of my album. I'm weird that way.
See the scans below