Without meaning to be picky about terminology, I'd like to try to clarify a few things here.
First, it really is not appropriate to use the term shelving when describing these filters. A shelving filter, or tone control, will raise or lower all frequencies beyond a particular point, by a fixed amount. For example, it might reduce all frequencies above 3 kHz by 10 dB. In contrast, the low pass filter has a roll-off slope beyond the set frequency, such that higher frequencies are reduced at the rate of 12 dB per octave (doubling of frequency). In other words, if the filter frequency is set at 3 kHz, then 6 kHz would be reduced by 12 dB, 12 kHz by 24 dB, and so forth.
Second, while these filters may be parametric in some literal sense (since you can adjust the frequency and Q parameters), they are really very different animals from what most people would think of when you talk about a parametric equalizer.
Here is exactly what you can do with one of these low pass filters and a three position Q switch:
The graph in the lower right shows what the Q switch does. In the high position, it gives you a sharp peak at the set frequency position, while in the off or 0 position it allows the filter to roll off smoothly without a peak, and the middle position gives you a modest emphasis. Note that in all positions of the Q switch, you still end up with a 12 dB roll-off above the set frequency. Q simply determines whether you get an emphasis, and how much, at that frequency.
Changing the frequency setting shifts the roll-off point (and the peak, if Q is engaged) right or left, as shown in the upper right. So as you lower the frequency, you are eliminating more of the highs, and more drastically than if you were turning down a shelving treble control.
