Erik,

You deserve more than the sound of the crickets after posting this “juicy”
obscure question.  So, in the interest of at least attracting more attention to
your quest:

Given the diameters you’ve measured, how did you eliminate size #4 and size
7/64ths as possible “nominal sizes?”

Sellers proposed/listed standard pitches for tiny fractional sizes including
7/64ths, but 50 tpi is not among them.

Have you considered the remote possibility that it’s Metric?  It’s “almost” a
pitch of 0.5mm.

IIRC, you have access and expertise on an Optical Comparator, which has become
an old tool in its own right.  And, your post certainly shows ability to perform
precision fine measurements.

“Grasping at Straws:” Can the thread profile be recognized as any particular
standard system?

Erik certainly provided impetus for wandering far out into the tall grass beyond
left field:  the 1959 re-definition of an inch was considered when pondering the
question of whether 50tpi is a round decimal in metric...no, that change only
shortened  the old US inch by two millionths of an inch!

John Ruth
“Of threads, and threading systems, there is no end!”