Hello MLM65 :
That is a nice find. It is a WW2 Type 99 sniper scope manufactured by Nippon Typewriter Company . This maker produced the first 2000 of the estimated 5500 scopes made in the 1941 - 42 period in Tokyo, the center of the optical industry.
It is a 2.5 power of magnification by 10 degree field of view, that is what is meant by the " 2.5 x 10° " on the scope tube, NTC Kogaku is the factory logo, and 99 No. 777 is the scope serial number.
As ' d - square ' mentions on thread #5, on the bright finish locking wedge on the dovetail ( on the reverse side of the locking lever ) there may be engraved a four digit number ( from 5000 to 7250, this is the rifle issue number ) or none. There may also be a one to two digit number stamped stamped inside one of those cavities, the middle of the lever shaft, the backside of the locking lever itself ( this is a production assembly number ).
Initially the scopes were mated with Kokura 99 snipers, these had the rifle number engraved on the outside of the mount area above the locking lever, so yours is definitely not one of those.
Many scopes in the No. 600 to 900 have been reported as "unissued", meaning that they were not mated to a specific rifle. Two such have surfaced in the past couple of days No. 707 and your 777.
Typically scopes in the No. 1000 to 1809 which is the highest serial number thus far recorded have engraved rifle issue numbers on the backside of the mount, these scopes were mated to Nagoya 99 sniper rifles in the 5000 to 7250 production block in late 1943 - 44. It is these issued scopes mated to Nagoya 99 rifles that should only be refer to as " Cutback " scopes .
The case itself is a canvas model of early shape same as the all leather ones of the 1930s, it is dated on the case opening as Showa 18 as arisakadogs and this translates to 1943. It appears to have the shoulder strap.
Regarding values, it depends on where the item is offered ( internet - auction - gunshow - trade papers ) and region of the country as well. The financial economy also enters the picture, if the scope is issued or not, and finally who is looking for one. Since only 1000 Kokura 99 rifles were scoped, these outside numbered scopes are the most desirable / holy grail. The 2250 scopes that were issued as cutbacks are the next wanted category. The other 2250 unissued scopes have value but are not as desirable as are the issued ones.
Once you let us know if there is a 4 digit rifle number on the backside of the mount, I will be able to tell you if the matching rifle exists or not in our files of known sniper rifles. If there is no issue number there then it is an unissued scope and it will be so recorded in the scope files.
Vicasoto