Hello, and thank you very much for your very detailed reply! This is one of the things that makes the internet so great, the amount of information available.

So while no Tu-134 'sans suffixe' was ever fully converted to a cargo configuration, all of them could carry a small amount of cargo. But you say that, among the Tu-134A's, only HA-LBI and HA-LBK could be converted for cargo flights. Was there a specific reason for this?

"Since a large cargo door was not produced on HA-LCA"

That might explain why there is no mention (at Russianplanes) of a conversion in the aircraft history, now that I think about it. It is however mentioned in a comment I just noticed. Although, if the floor was reinforced... Hm. I'm not quite sure where they draw the line for what counts as a conversion and what doesn't.