Hi there Damir -
We followed up on this since I never received a response and found out that there was an error along the way. Bell's parent company, Diageo did send an email to you - did you get it? Regardless, this is what we found out:
Using the photographs you provided, Diageo's Archive Department attempted to date the bottles. The bottle hat has the front label pictured has a features that match bottles from the late 1950s but other features that match bottles from the 1960s and 1970s so they believe it is from the early 1960s. The back label that was pictured matches the label from bottles from the early 1960s to mid 1970s.
Diageo's Archive Department may be able to date the bottles more accurately - but they need you to answer a few questions. They only have two pictures, one of a front label and one of the back label. Are these from different bottles or two shots of the same bottle? Are both bottles the same? If the bottles do differ significantly additional photos may allow them to be more precise with the bottle dates.