5 dollar yard sale 2-57 500 Set with interesting Markings on the bottom

[Hill Haven Telephone Company]

Picked this 500 set up at a yard sale for 5 bucks down the street from my house

Has some interesting markings on the bottom.  Any idea what they are for? 

[paul-f]

The S- codes are refurb dates.

500 was stamped over 501 when the set was converted and the tube was removed.

The handwritten numbers are probably user or installer notes. Similar four-digit numbers on other sets have been the last four digits of the phone number the set was to be installed on or was removed from.