Title Rastogi is used by two communities in India, as per my knowledge. One belongs to the group of swarnkar or sonar and the other is Baniya.
Though both communities accept the same origin of the title but the two don't accept that they are same community.
All sonars do not write Rastogi as their surname or title. There is a vast number of subcaste among sonars.
It seems that there were peoples from many castes who were earning their livlihood through this work of craftmanship, i.e. the art of making jwellary out of gold. In the long run all these workmen were given a common name i.e. sonar. Actually sonar is not a single caste, It is a club of the families involved in the same occupation of making jewellary of gold and earning their livelihood. The families have joined this club from different castes. For a layman sonar may be a caste but it is actually the name of a club and there are the people of many castes in this club. I have seen the people writing themselves Brahmin Swarnkar or Brahmin Sonar.It is very clear that when someone of this family's ancestor left the work assigned for Brahmins and joined to this work, was not accepted as brahmin and was forced to call himself Sonar. Similarily there are kshatriya sonar, gupta sonar and may other castes too.