According to the webpage linked to below, T-tops first appeared on the a-body in the Cutlass based Hurst Olds in 1975. And even then, it wasn't factory. It was a Hurst conversion. So, t-tops weren't available for the 73-74 model years at all. Then, once t-tops became available and could be ordered from your dealer, there were never any factory made and installed t-tops on 75-77 a-bodys. The cars were sent to a Hurst facility to be converted.
So, a 74 could not have originally come with t-tops when it was new. They would've had to have been added later, and are likely Hurst.
http://www.pontiacsonline.com/Those%2070's%20T-Tops.htm