File H1B transfer with approved i140 Receipt number
You can use your approved i140 receipt number to file H1b transfer or extension after 6 year quota has exhausted.
Employer withdraw i140 after 180 days
Your employer can file for withdrawal of i140 when you leave them but USCIS will not revoke it if you have stayed with them for at-least 180 days after the i140 approval.
You are fine to keep using this approved i140 for H1B extensions.
Request i140 Information from USCIS Directly
You can request your i140 information directly from USCIS using FOIA service.