I have some 3ml syringes with 23Gx1" needles. But there is also a second scale on the syringe that is marked as "M" and 40M is right at around the 2.5 ml measure. What is this M scale mean? I thought it might be IU but the numbers wouldn't be right.
Minim (unit): Part of the Apothecary system. It is rarely used anymore(some countries and areas still use the apothecary system) and is a alternative to the drop(which was formerly the smallest unit of the apothecary system).
The minim (abbreviated min, ♏ or Mx, a symbol for minim in the apothecaries' system.svg) is a unit of volume in both the imperial and US customary systems of measurement. Specifically it is 1⁄60 of a fluidram1 or 1⁄480 of a fluid ounce.
40 minim are equal to 2.48 ml. Varying slightly on what kind of calculator you use.
Since its a Injection you would round the number to the nearest 10th, so 2.46 or 2.48 would become 2.5ml's.
There appear to be a few types of the minim. For this, I only chose the calculation for the mimim that plainly said Minim, there appear to be a imperial and US or UK version of the Minim.
Picture courtesy of cwladis.com