A useless thought: in FORTH, they'd be using the /mod operator. That's a division operator that returns both the (integer) quotient, and the remainder. Just look at the pattern:

Roman	Factor	Exponent
I	1	0
V	5	0
X	1	1
L	5	1
C	1	2
D	5	2
M	1	3

The factor clearly has a repetitive pattern, with a period of 2, the exponent increments once for every period.