"EIN # of the limited partner, who is registered as an LLC with the IRS." 

How does an individual register  as an LLC with the IRS?

Did this person form a SMLLC?

"client who is the only General Partner of a Partnership and his partner is the Limited Partner, however, since they registered the company 3 years ago," Registered where? As in set up a General Partnership with the state corporation/entity division?

I am confused.