This issue (U586) is a challenging one and a delight for the EFO collector.
There are three general types of surcharge errors:
(1) operator placement error, where the operator loaded the envelope in the cancelling machine (a) inverted at lower left, (b) on the reverse at the upper left, or (c) on the reverse at the lower left.
(2) feed mechanism slippage error, where the rubber rolls that are moving the envelope left and right cause the envelope to skid and are out of place when the surcharge is applied.
(3) no surcharge is an error because ALL U586's should have a surcharge and an envelope without one is therefore an error.
Types 1 and 3 qualify as major errors and appear in the UPSS catalog at anywhere from $200 to $750.
The OP's envelope is a type 2. However, there are two attributes of OP's envelope that make it more desirable than its siblings: (1) the surcharge strike is almost perfectly SOTN, and (2) it is used.
This specific EFO type is not mentioned in the catalogs, but if it came up for sale in eBay
, I would set my snipe at $50.
[bear in mind that I am slightly crazy when it comes to EFO's]