Candidate

this particular coin is in bad condition;

there is no proof that a date was altered on this particular coin;

in my book second edition it is listed as 25 ye;

in real life, who and where in Russia reported a counterfeit of this coin?

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.

it is very much doubts that who wants to counterfeit absolutly unrare coin in such condition?

just a little years ago these coins were available in brooklyn at 50 cents or less each,

and there were better condition that this one of 1970,

also by the only one I believe authorized work by Fedorin this one has a variey which estimated at 25 y e

but I do not know if this is a variant presented by you

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.

Couldn't attach larger image...

A popular issue, very competitive pricing these days!

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...)