Candidate

Anyway, statement that it is "absolutly unrare coin" is not correct$ it is not "rare", but scarce ("key date") and popular. Also, even in your "book second edition" (2004) there are no varieties of this year listed - so there are no other "variants", which you had mentioned. And "this particular coin" would be "about VF" (imho) and still cost $100~$150, if genuine (but most probably it is not). Counterfeits of other scarce coins of 1961-1991 period (as 20 kopeks 1991 w/o mintmark, 20 kopeks 1976, etc.) are informed about quite often; I noted such reports in web-forums, etc. - even though I'm not really interested in coins of this period.

I'm afraid you are not well informed in this case (at least less informed than about past auctions catalogues). In the "only-one-you-believe" work by Fedorin (3rd edition) there are no varieties of this coin, only one main type (Fedorin listed it at "50 ye"). 5 kop. 1970 is the most scarce date out of 1961-1991 series, and it's current market value is around $150-$200 in VF-XF condition. So there is some reason to counterfeit it: simply in order to transfer "50 cents" (actually, even less than that) into $150.

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And lots of fakes too... I'd suggest that you also show your coin's edge - and then ask some knowledgeable person to give his opinon... I'm not an expert in this type, but it looks a bit suspicious to me... but maybe just well cleaned... (All imho and sorry if it is a false alarm...)