O-suriage is a blade that is so much shortened as the original tang is cut off. That doesn't have a part of original tang nor signature. The tang is completely re-shaped. On the other hand, the tang which has a part of original tang left is Suriage. O-suriage means large suriage.
(=> Manner of SURIAGE)
When we meet tachi blades from Kamakura period or Nambokucho period (12-14th century), most of them are shortened. Some are suriage and some are O-suriage. In other words, that old tachi blade with original length is very rare. Such long Tachi blades became inconvenient in the later period, so they were shortened to accord with the new style as Katana. Suriage blades have a part of original tang or signature. It is a hint to imagine the blade in original. But, O-suriage blades are more difficult to do.
On the other hand, we find many un-signed blades those can be confused to O-suriage. Some are originally un-signed, and some are removed their signature somehow.
Now, let's study O-suriage blades. Is it real O-suriage blade, or just removed inscriptions in the tang?
At first, studying the shape is very important. O-suriage blade must be shortened so much, at least 10cm and more. So the shape is changed from the original. The fumbari must disappear. The shape looks that the base is not wide enough while the middle of the blade length is wide. When you hold it vertically, you may seem it like big belly. It is strange as an original shape, but very correct as O-suriage blade. On the contrary, if there is a neat shape on O-suriage blade, we have to study it carefully. How is the fumbari? If it still has big fumbari, it may be not much shortened.
(=> Fumbari)
Next, study the hamon around the machi. The original tang is cut off, and the new tang is made from lower part of original blade. Therefore the hamon runs into the newly shaped tang, because the start of original hamon is deep in the tang. Sometimes, hamon disappears by the heat of shortening work. In such case, the hamon is vanishing around the machi.
If the hamon starts around the machi, it is not a correct O-suriage. The blade may be original length or re-hardened.
Please study the utsuri also. If the utsuri starts at near the machi, it is not much shortened. Because the start of utsuri is a start of hardening effect.
Please notice that the vanishing of hamon around the machi is not a proof of O-suriage. It can be happened on original length blades.
On the way of blade smithing, it is usual way to heat the tang again after the hardening work (yaki-ire). Because the tang can be hardened by the yaki-ire work somewhat. The heating of the tang puts it back in even mildness. It is necessary for the later work, finishing it with file and inscribing on it. Sometimes the heating makes the hamon vanishing around the machi.
For example, tanto blades in 13th cenetury have such an appearance around starting of hamon sometimes. And Bizen blades in 16th century also have such appearance commonly. Probably smiths didn't design the starting of hamon carefully, since it was a weapon.
Why the signature on original blade is removed?
We can find several reasons. One is tricking people. Unpopular smith's signature can be removed to confuse with O-suriage blade of popular smith. This kind of business is supported with some certification. The work begun in 16th century, and it continues today. It is not a counterfeit, but a matter of certification. We just study blade quality.
Examples of such fabrication we often meet
Sekido => Ichimonji
Echizen Yasutsugu => Masamune
Mihara => Rai
(=> Episodes by Kokaji, an O-suriage katana "Shizu")
In old days, good swords were also used as the way of rewarding the man who worked well. We heard that some incorrect O-suriage blades were made for that purpose, when the big names ran out. It may have some truth.
Sometimes, fake signature or wrong inscription on the tang is removed. In such case, the tang is not shaped as O-suriage.
On the other hand, we can find a really good blade with a trace of removing signature in the tang. The blade is original, and so is the tang. But the signature is removed. It is hard to be understood why that was done. The work has destroyed the originality of the blade.
There is a story if it may be the answer.
Once there was a shogun (head of samurai society) who loved sword very much. He researched all the lords what swords they handed down. Some load was so scared if his heirloom sword would be taken away by the shogun. Then he removed the original signature on the tang, and reported it as just an un-signed blade.
Sometimes another kind of shogun also appeared who loved beautiful woman. Lords hid their wife deep in their room.