I’m a little confused by the replies.

My handlebar ‘s OD appears to be constant (no tapering to speak of) excepting a very small area positioned directly underneath, and gripped by, the stem: the latter section doesn’t extend past the width of the stem for more more than 1/8” on either side.

The overall width of my handlebars are about 42cm: you're saying the 24mm Tektro lever fit my bar? I'm not comprehending how the handlebar width impacts the size of brake lever, or where it is mounted?