Occasionally South Korean stamps show up on popular (stamp) auction websites with a 10 won overprint on them. But are these overprints real? According to John Sauer in his Monograph 1, Specialized Postal Stationery Catalog of the Republic of Korea, page 7, postal card KPC #PC15 was created with a 10 won surcharge on the old 5 chon card. It was first issued on 1949.5.3 and was superseded by a 20 won surcharge on the same card design on 1950.5.1 (designed at PC16). Luther Dilley’s Korean Stamp Catalog (Part V in our online offering to members) on page 72 identifies these cards as PC3 and PC4.
The overprint therefore was genuinely applied on those cards. But how about the stamps? The first group of stamps bearing such surcharges appeared sometime in the early 1960s. They were mentioned several times in KP in 1961-1962. Here are the relevant texts from those KP’s:

(From KP Vol. X No. 2, May 1961)
Mr. R. L. Frock from Colorado Springs, Colo. has submitted a group of 30 surcharged stamps, some of which are shown here. These stamps have the surcharge of the revalued postal card pictured on the cover of KP (the fourth illustration from the top on the right hand side, — on the postal card the surcharge is in red ink.)
The surcharged stamps Mr. Frock sends include Scott’s nos. 78, 93, 98, 100-105, 109-113, 121, and C2. The surcharge is found in black and purple. Thus, we have one C4 with the black and one with the purple. Some are mint and some used. In the used the strike is carefully placed on the edges and the post office is the same and in Chinese characters.
Mr. Frock secured these stamps from a dealer in Seoul, and would like any information he can get about them. Are they postal favors, or were they a valid issue?. He reports that originally he had some of them on cover but took them off.
We know of the use of this surcharge for revaluation of the 5 cheun, red (type A4 -Scott) postal card. Further inflation resulted in similar surcharges on this postal card of 20 won and 50 won values.

(From KP Vol. XI No. 1, February 1962)
The Provisional Postal Card Surcharge on regular stamps was reported and illustrated in the May, 1961 issue of KP by Mr. Richard L. Frock #254, Has any other member of KSS seen these or have them in their collection? The surcharge is the same as the 10 won revalued postal card, KSS # PC 3 of the Specialized Catalogue published by Mr. Luther Dilley. Any information about these provisional surcharges on regular stamps would be appreciated by Mr. Frock and your editor.

(From KP Vol. XI No. 2, May 1962)
The PC3 Surcharge on Stamps
Since the discovery of regular stamps of Korea surcharged with the PC4, 10 won revaluation, we have attempted to determine the validity and source of these stamps. No definite information is available as to what their status is. Bill Rhee and Lyman Hale, in Korea, report that they are unknown over there.
Yet, quantities are appearing and the KSS has been requested to verify these stamps, if possible.
The variety and extent of these is apparent from the following listing. Scott’s numbers are used. The letters B (blue) and P (purple) indicate the color of the surcharge:
61B 74B 97B 105B 118B 62B 74P 98B 105P 118P 63B 75B 99B 109B 119B 64B 76B 100P 109P 119P 65B 77B 101B 110B 120B 66B 77P 101P 110P 120P 67B 78B 102B 111B 121B 68P 79B 102P 111P 121P 69B 87P 103B 112B C1B 70B 93B 103P 112P C2B 72B 94B 104B 113B C2P 73B 104P 113P C3B
Japanese Telegraph Revenue Stamps are known with this surcharge.
Under what circumstances these were used, if valid, is not known. At this point it would appear doubtful that they are legitimate since they are unknown in Korea and there are none known on cover.

Of course, when they first appeared in/around 1961, who was to say? Nobody had seen them before. However, since then it has become an established fact that these overprints on these stamps are utterly bogus. Who made them is not known, but they may be of South Korean origin, as that is where Mr. Frock obtained them. Who knows more about these stamps? Please leave your comments below.
The shown korean stamps and the japanese period telegraph paper seal* all show handstamp strikes, not overprints.
The handstamps text means “adjusted value 10 won”. It is a type exlusively used on stationery cards 1949/51 ( KPSC postcards PC15/18). The original handstamp type stating 10 w. had a diameter of 23mm and was used on the card PC15 only, sold from 1949.5.3. It was placed over the obsolete imprint of 50 Ch. Later issues had higher values 20 w. ec. It was a special type for stationery and complete nonsense on postage stamps.
Korean posts used this and similiar types on cards and other stationery inc. airletters until the 1980s.
These types were never used for revalueing postage stamps.
I concur that they are utterly bogus.
*Not a revenue, a seal used for sealing telegram slips which were folded and delivered as letter, so called “letter telegram”.
Another example of a bogus 10 won overprint showed up, this time on Kobay, a Korean language auction website. The seller asks 195.000 won (approx. 150 EUR). What this shows is that apparently even within South Korea there are stamp collectors who don’t know these overprints are bogus, or else there wouldn’t be a market for them within Korea. That is if the seller manages to sell this item of course. (To be honest: I did do business with this seller quite some time ago and I didn’t have any complaints at the time.)