A. Newman and Co., job and postmaster, 121 Regent Street (i). The OED gives defines jobmaster as "a man who lets out horses and carriages by the job or for a limited time". It gives one meaning of postmaster as "The person in charge of a posting station, who provides horses for posting; the proprietor or manager of a posting establishment". It also gives one meaning of job as "A cartload; the amount that a horse and cart can bring at one time".
References:
(i) Ref: The Post Office London Directory 1843 p307