After 23 years this beast has given up the ghost. Unfortunately, no one seems to know that this beast existed in the first place. It is a Honeywell Aquastat L8148L-1005 (please note the "L" after the 8148 as opposed to a "A", "E" or "J". I've checked the web, local plumbers and local supply shops for a replacement without anyone having heard of the "L". I checked with Honeywell but in keeping with policy they won't provide me with any information. One supply house thought maybe it was a Canadian model but you would think it would show up on the web somewhere regardless.
It's appearance (see attached photos) is very similar to the L8148A, which is a horizontal mount and mine is a horizontal mount but is mounted vertically. Go figure! The system is quite simple: 2 zones on a gas-fed Burnham Series 2 boiler. Of course I've had all summer to solve this but here I am now facing a chilly autumn.
If anyone knows what the correct replacement for the L8148L-1005 would be I would be most appreciative.