Additional information on this issue is provided by Stallbaum in his 1820 preface. There, Stallbaum tells us that Heindorf made numerous emendations in marginal notes to his copy of the Bipontine edition of Plato which was given to him by August Jacobs. Heindorf had recorded in the margins not only notable variant readings but also all the corrections he had previously proposed—published in a collection of conjectures on Plato or shared with Schleiermacher. Later, apparently upon rereading Philebus, Heindorf added new conjectures in the same volume.
Source:
Stallbaum (1820: vi–vii): Tanto gratius mihi accidit, quod virorum summorum, Schützii, Heusdii, Heindorfii, Schleiermacheri, Baumgarten-Crusii, Beckii, Astii, Bekkeri, aliorum, opera in tollendis libri vitiis ante me erat versata. Quorum acumini ac diligentiae quantum Philebus meus debeat, lectores ipsi facile iudicabunt e commentariis textui subiectis.
His auxiliis accesserunt aliquot praeclarae Heindorfii emendationes, quas acceptas refero benevolentiae erga me singulari Augusti Iacobsii, Professoris Halensis celeberrimi, Regii Paedagogii Inspectoris meritissimi. Qui, quum exemplar Platonis Bipontini, quo olim usus fuerat Heindorfius, possideret, a me rogatus insigni liberalitate, si quid ex Heindorfii notis mihi prodesse posset, id lubenter mihi utendum concessit.
Notaverat autem Heindorfius in margine praeter lectionis discrepantiam insigniorem emendationes omnes, quas olim vel in specimine coniecturarum in Platonem proposuerat vel Schleiermachero communicaverat. His serius, ut videtur, novas addiderat coniecturas, repetita Philebi lectione enatas. Quarum nonnullas, utpote Heindorfio indignas, silentio pressi; alias, suo quamque loco, addito auctoris nomine memoravi.