Hi, thank you for looking into it.
28 has never been it's serial, rather a code added later by the museum. It is normal for museum crew to change the numbers and liveries in Bulgaria. Scramble reports it as 72 here: https://www.scramble.nl/database/military/bg#results
While 28 is only listed as "photoproof" here: https://www.scramble.nl/database/military/bg#results
As they are probably also confused about why new registrations just appear out of the blue. I already provided this information when the photo was submitted, as you submit by actual registration and not painted ones...
Also here, when looking for wrecks and preserved planes, it is logged as 72: https://www.spottingmode.com/wro/bulgaria/planes/
It's registration is indeed 72, and as far as I can see, it is the first photo of this aircraft on DB (Even if searching for 2.
Thank you ...