Can you claim retroactive SRECs? Yes.
These are a few variables that effect their ability to be purchased.
Produced but not brokered: If there are existing RECs that have been produced but not brokered, those will be included in your contract in the option you select.
Depending on age and contract type: Previously produced SRECs may be sellable depending on how old they are, as well as the contract type you select.
Age: The price may be lower for older SRECs as typically these "expire" sooner than newer SRECs, as a result their utility is lower and that is reflected in the market price.