How Sivalinga Sculptures have changed through history:
I have noticed that Sivalinga sculptures were made somewhat differently in our long past (atleast in the northern region of India). It was customary then to sculpt a deity figure on the lingam itself. In our present times, this practice of sculpting a deity figure on the lingam has been replaced by drawing 3 lines on the lingam and a dot or nothing at all. This practice of sculpting a deity face on the lingam continued for a long time i.e., from Kushana say upto 7th century or so (as per my observation so far). However, the present day Sivalingams are not made with a deity face on it.
The first photo below is of the Kushana period (approx. 2300 year old, Mathura region) sculpture of Sivalinga showing deity image on one of its sides.
The second photo is that of Gupta period (approx. 1800 year old) Sivalingam sculpture showing a group of 5 lingams (one in the centre and 4 around it) with faces sculpted on them.
The 3rd one is an undated sivalinga excavated in village Kiloi (dist. Rohtak, Haryana) showing deity face on the lingam.
The 4th one is a contemporary styled sivalingam (with no sculpted image on it).