No you're not lost, this is not a misplaced blog post from years ago. Recently I've needed to export a Exchange 5.5 directory and since I wasn't able to find my copy of header.exe I had to do some digging to find the list of available header fields.

Since it took me a while to find them, I thought someone else may need them as well, so here we are, the complete list of headers for mailbox/Gal exporting in Exchange 5.5

Obj-Class,Delivery-Mechanism,Directory Name,Accept messages from,Accept messages from DL,Address,Admin Note,Admin-Display-Name,Alias Name,Allow rich text,Alternate Recipient,Assistant,Assistant phone number,AutoReply,AutoReply-Message,AutoReply-Subject,Business phone number 2,Business-Roles,Can send on behalf of,City,Company,Country,Custom Attribute 1,Custom Attribute 10,Custom Attribute 2,Custom Attribute 3,Custom Attribute 4,Custom Attribute 5,Custom Attribute 6,Custom Attribute 7,Custom Attribute 8,Custom Attribute 9,Deliv-Ext-Cont-Types,Deliver to both,Deliverable Information Types,Department,Description,Display Name,E-mail Addresses,Expiration-Time,Extension-Data,Extension-Name,Extension-Name-Inherited,Fax number,First Name,Heuristics,Hide from AB,Home phone number,Home phone number 2,Home-Server,Imported-From,Incoming message size limit,Initials,International-ISDN-Number,Issue warning storage limit,Last name,Maintain-AutoReply-History,Manager,Master-DSA,Mobile number,Notes,Obj-Admins,Obj-Container,Obj-Dist-Name,Obj-Users,Office,Outgoing message size limit,Owner-BL,Pager number,Period-Rep-Sync-Times,Period-Repl-Stagger,Phone number,Postal code,Primary Windows NT Account,Prohibit send storage limit,Reject messages from,Reject messages from DL,Reps-From,Reps-To,Reps-To-Ext,Secondary-Proxy-Addresses,Security-Protocol,Simple display name,State,Telephone-Number,Teletex-Terminal-Identifier,Telex-Number,Title,Trust level,Use IS defaults,User-Cert,X121-Address,X500-Access-Control-List

For a more complete reference (I know, I know, the fun doesn't stop) grab this download