Definately imperial!
In a former life I was a fitter & turner; in the trade, a bee's dick was 'half a thou' (ie. 0.0005") however the use of the term was expanded to include any amount that was 'less than a thou'*.
Perhaps Frant was a boilermaker? (boilermakers having, over time, adopted their own approach to measurement & tolerances).
Any serious discussion on this matter would not be complete without mentioning another imperial unit, the "c$$t hair".
This unit is a distance equal to 'two thou' (ie. 0.002") (plus or minus a bee's dick).
Those of you with access to both a micrometer and an actual "c$$t hair" will be able to verify this.
Note: both the "bee's dick" and "c$$t hair" are singular units, and as such are only used by themselves and never in the plural sense.
You would NEVER refer two "two bee's dicks", or "three c**t hairs"; that would just be silly!

I trust that this helps clear things up.
*Reference: Page 2435 MAchinery's handbook, 25th edition, 1996 Industrial Press Inc