[cfuruti]

My guess is that 1/33 is a close enough approximation of 1/33.333... = 0.03.
In other words, the designer can shortcut a division by 33(.33) with a multiplication by 3 followed by a division by 100 (obviously trivial with decimal units).
Today, computers can almost make us forget about arithmetic, but in ages begone the choice between "x 3" and "/ 32" could favor the former.
That's just my speculation, since slide rules make approximate divisions by either 32 or 33 equally easy (I could envision a seasoned designer notching or painting a tally at his/her favorite scale...).

BTW, the old "33 rpm" turn speed of LP vinyls is an abbreviation for 33.333..., or 100 rotations in 3 minutes. Similarly, perhaps pre-calculators "1/33" models are actually 1/33.333...?