H1B Interview Waver (From L1 Blanket to H1B)

Thanks for sharing timely updates and it is helpful alot.
My Question: I got the L1B approved visa on 20-DEC-2017 and that L1B visa got expired on 18-DEC-2020. I got the H1B on 01-OCT-2020. Recently I got the extension for the H1B Visa from 01-DEC-2024 TO 30-NOV-2027.
I have calculated the 48 months from the date of L1B expired(18-DEC-2020) is " “December 18, 2024”.
Question 2: Here the 48 months are calculating from the L1B expired visa or before extension existing H1B valid visa. Could you please suggest on this. Thank you.
Today i am planning to book the appointment, so am i eligible for the DROP BOX or not could you please confirm on this. Thank you.