GST Invoice Series for different services

As you are already registered under GST Delhi as M/s ABC Services (Proprietorship) for providing Security services, you can issue invoices for rental services under the same GST number.

However, you need to declare the rental income in your GST returns and pay tax on it. 

Since the property is in your personal name, you may need to obtain a separate GST registration for the rental income, depending on the taxability and threshold limits. 

Regarding the invoice series, it's recommended to use a separate invoice series for rental services to distinguish them from your security services.

 This will help in maintaining clear records and avoiding any confusion.

You may use a prefix or suffix to differentiate the invoice series, 

for example: - ABC-RNT-001 for rental services - ABC-SEC-001 for security services Remember to mention the GST number and other required details on both types of invoices.