If that information is anywhere it's in the 'moodle_backup.xml' file of an extracted backup. In this case, think 'format' is a little misleading as version 2.3 and 2.4 have the same course formats - no difference.
Format in this instance might refer to version of Moodle.
In an extracted 2.3.4+ course moodle_backup.xml file:
<information>
<name>backup-moodle2-course-73-testing-20130224-1442.mbz</name>
<moodle_version>2012062504.06</moodle_version>
<moodle_release>2.3.4+ (Build: 20130214)</moodle_release>
<backup_version>2012062500</backup_version>
<backup_release>2.3</backup_release>
<backup_date>1361734965</backup_date>
<mnet_remoteusers>0</mnet_remoteusers>
<include_file_references_to_external_content>0</include_file_references_to_external_content>
<original_wwwroot>http://URLtomdlhere</original_wwwroot>
<original_site_identifier_hash>36451af7276e5ebdca32defccbb33e7f</original_site_identifier_hash>
<original_course_id>73</original_course_id>
<original_course_fullname>testing</original_course_fullname>
<original_course_shortname>testing</original_course_shortname>
<original_course_startdate>1361682000</original_course_startdate>
<original_course_contextid>1741</original_course_contextid>
<original_system_contextid>1</original_system_contextid>
<details>
<detail backup_id="ff5eeef25a2f6dc92f044e7da109c4f5">
<type>course</type>
<format>moodle2</format>
In a 2.4 backup moodle_backup.xml file of a course set to social format for the course (don't get confused below ... the word 'social' below was course short name ... not really the course format):
<information>
<name>backup-moodle2-course-1678-social-20130328-0407.mbz</name>
<moodle_version>2012120303.01</moodle_version>
<moodle_release>2.4.3+ (Build: 20130322)</moodle_release>
<backup_version>2012120300</backup_version>
<backup_release>2.4</backup_release>
<backup_date>1364465259</backup_date>
<mnet_remoteusers>0</mnet_remoteusers>
<include_file_references_to_external_content>0</include_file_references_to_external_content>
<original_wwwroot>URLtoMoodleHere</original_wwwroot>
<original_site_identifier_hash>dadf2ba034a4f6e2d19749bade889b2a</original_site_identifier_hash>
<original_course_id>1678</original_course_id>
<original_course_fullname>Social Format</original_course_fullname>
<original_course_shortname>social</original_course_shortname>
<original_course_startdate>1364536800</original_course_startdate>
<original_course_contextid>1807</original_course_contextid>
<original_system_contextid>1</original_system_contextid>
<details>
<detail backup_id="ff6ff8bc535477f536576ca1d15d2fbc">
<type>course</type>
<format>moodle2</format>
One might be able to change information in the following:
<moodle_version>2012120303.01</moodle_version>
<moodle_release>2.4.3+ (Build: 20130322)</moodle_release>
<backup_version>2012120300</backup_version>
<backup_release>2.4</backup_release>
to something for a like backup from your 2.3 and satisfy moodle checking for 'format'.
'spirit of sharing', Ken