IMHO and experience - The 1898 Krag rear-sight (Dickson sight) is unusual, but, will show up at larger gun shows that have an amount of Krag rifles and parts.

The carbine version is rarer than the rifle version, because fewer were made. However, a large number of the rifle sights were taken apart to salvage and grind the 'leaf' to make a variation of the 1902 sight.

'Dickson' (1898 rifle) bases are very common in 'loose form' on Ebay.

Many incorrect sights, that are 'marriages' of m-1898 'bottoms' and m-1902 'tops' are around, that were probably assembled by surplus dealers or 'Hobbyists'.

I have seen the Dickson carbine sight priced or 'selling' for around $200-250 in nice condition. The rifle version is probably a $100-150 item.

These are of interest mainly to collectors who have a special interest in sights. They are not 'Hot Sellers'. 'Condition' is very important.

Your Dickson sight is correct and complete, but, on the rough side. I would estimate it as being worth about $100.

FWIW - I believe the Dickson sight would have only been used originally on model 1898 rifles and model 1899 carbines that fell approximately in the 218K to 253K serial number range. Probably, most of these arms received later sight 'updates'.

Creative things were certainly done by Surplus Arms Dealers in the 1920s and 1930s. U.S. Armorers may have done creative things during WW1, when Krags were training and stopgap equipment.

A photo of the model 1902 sight variation using salvaged 1898 leaf (note - someone 'unofficially' has put a washer under the windage knob):

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