A lot of the Sphere mechs that were in the 3050 TR are all quite old, most of them go back to the height of the Star League, so almost all the pre-3050 designs are options. That was one of the nicer things about early Battletech, they didn't feel the need to make up new (and often very, very terrible looking) designs to fill endless books when the new mechs have essentially zero difference to ones that came out in the first few tech readouts. Tech readout text itself even says how rare new designs actually were, only to have dozens of them pop up over the next few years.
Sarna.net has all the tech readouts online, brings back memories going through the older ones.