The first Parkers were hammer guns from about 1867 and the hammerless was introduced in about 1887. The hammer gun was continued until about somewhere soon after WW I. The first hammer guns were of the T-Latch design but were soon replaced by the lifter. Then came the top action but it had a straight lever and the gun could not be opened without first decocking the right hammer. The "fishtail" or serpentine top lever was developed to allow the gun to be opened safely without the sometimes unsafe decocking of the right hammer.
This is the simple condensed version - others will add details.
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