PART NO.  SPL.PAGE  DESCRIPTION                                                           
88G257    16.A.9    Plug -drain -(oil reservoir, engine, 1600)
88G257    16.A.30   Plug -drain -(oil reservoir, engine, 1622) -part no. changed, was
    2F4249 and 2A380
2F4249X   aftermarket  Plug - drain - oil reservoir -magnetic

Drain plug for Austin B-series engines, 1953-1980 (except Twin Cam).
The magnetic plug is an aftermarket accessory, not an original factory issue part number.
Part number was changed from 2F4249 to 2A380 then 88G257.
Thread size is 3/4-14-AFT (Admiralty Fine Thread). This is a Whitworth thread form with 55-degree thread angle, 0.747" major diameter.
Use with 6K638 Washer for plug (3/4" copper crush washer).

Here we have an idea from Christopher Tunnicliffe in Wiltshire, UK. He writes:
"I use JB Weld to glue little strong magnets to my engine/gearbox/axle plugs, and these seem to work well. The strength of those magnets may even make the JB Weld an optional belt 'n' braces measure".

This looks like a cheap work around for some expensive aftermarket parts that may or may not exist.