I acquired the item below off the Internet - it is a wrapper with a "MA**OCK / 126" boxed mileage mark addressed to Ludlow, with a filing date of 16th July 1803 and a postal charge of 7d.
Here is an expanded view of the handstamp.
One can see the "M" and probably the "A" then there is a gap with a vertical serif, followed by the "OCK".
There are two places that it could be from - Matlock in Derbyshire and Martock in Somerset.
The vertical serif fits with Matlock but the mileage, 126, fits with Martock (Matlock mileage to London was 146). The postal charge of 7d fits with Matlock, and Martock to Ludlow could be the same rate if the postal route was very direct.
However, 126 was the mileage for Derby. There is a strange anomaly, with the mileage marks for three towns north of
Loughborough; Kegworth, Cavendish Bridge and Shardlow all of share the
mileage 109 for Loughborough. Derby mileage was 126 so is this a handstamp for Matlock with the Derby mileage ?
In either event, it is a previously unrecorded handstamp.