Past progressive after the past perfect (follow-up question)

[Rustamsher]

(a) I had lost my keys when I was going to the post office.
(b) I lost my keys when I was going to the post office.

Dear Alan, as far as I know both of the sentences are grammatically correct.

However, I have two questions:

1) Are they semantically identical (i.e, do they mean the same)?
2) Which sentence sounds more natural?

Thanks.

[Alan]

1) In most contexts, yes.
2) Either is fine.