Resistor value labeling, what does the second number refer to?
Most likely the 'K' position indicates the decimal point position. 

So,
$$ 5K1 \equiv 5.1k\Omega $$
$$ 3K9 \equiv 3.9k\Omega $$ 

In the same vein as:
$$ 3V3 \equiv 3.3V $$ 

Just a way to save characters/area, and avoids the issue of the decimal point fading out when printing/copying/faxing.