Yep. RSK MiG is the outfit that makes these jets. The Socialist Party government awarded them the contract years ago while briefly in power.
Is Bulgaria a NATO country or an affiliate?......
Even if Bulgaria had any other option,they would still have had to get RSK MiG involved.The Russians are increasingly cranky when other (primarily Israeli & other European) companies win upgrade offers for their products-they usually show their displeasure by shutting off supply of spares.