As much as cassettes need a carrier, it works the other way around too. Cassette carriers are built to handle multiple spark bonds with their symbiotes, and without those bonds their spark can get overwhelmed with all the extra energy and start damaging itself, worst case, it could even burn out if it doesn’t form a bond.

So when Blaster got his cog, his spark immediately pushed him to start carrying. That’s how Rewind and Eject, his twins, came about. Unlike most other frame types, cassette sparklings grow up fast, depending on their size and resources, they can hit their full frame in as little as 5 to 7 vorns.

So barely after 6 vorns of having the twins, once they reached full size, Ramhorn and Steeljaw followed after soon in a second carriage. (For clarification, Ramhorn and Steeljaw are only brothers, not twins. Twins are split-sparks, not those who came about in the same carriage).

This takes place even before the main events of the main baby prime AU, so by then this cassettes were already out and about.