Recently I was weighting stuff with my dad and as I was playing with one of the old 2 kg weights I felt with my fingers number "5". After cleaning and upon further inspection it turned out the writing on the weight says "5Φ". We checked and it indeed weights 2 kg as my dad remembered. It was done with a simple balance scale by comparing with another weight of similar age that is properly marked as "2 kg". There are no other marks on that weight. On the opposite side is a bulge but it's an irregular blob - I think it's remainder of sprue.
So, question that arised in my head is - what does that Φ mean? Is it some rare unit of mass? Some sort of preferred series or standard for weights?
I was looking for answers and found nothing. I ruled out pounds that was first thing that came to my mind because the difference was to big (5 pounds is approx. 2.26 kg) - though there are some signs of wear and corrosion it's in rather good shape and I don't believe it weighted 0.26 kg more originally. Also, considering I'm from Poland and the weight is really old it probably was made during the soviet era (maybe even in USSR) - so that phi could be actually cyrilic fe - but that also gave me no results.
As a last resort I decided to ask here - maybe some member of that community will know the answer. Below is a photo of the weight in question.