What kind of oil plug is this? (LAC)

[Tony Lum]

Hi gang,

When I first got my 5kcstq, it came with a special kind of oil drain
plug.  I can't figure it so I'm asking for the list's advice.  Its a normal
hex shape stamped "ARC INC" with "PATENTED" underneath (no number
though).  Past the 14Mx1.50 thread there are 3 small holes with a spring
visible inside.  The end has a captive brass end that look likes a temp
sensor.  The top of the bolt head has a flat neoprene cap that you can lift
up but won't come out (at least as far as I've pulled).  Appreciate any
knowledge of what this gizmo is.  Right now I replaced it with a stock oil
drain bolt.

Regards,
-Tony
1987 5kcstq, 1.8 bar