Not fair, Steve B, you've been having all the fun
It all looks very "FM" so not hugely surprised it's exploded out through the shell. The FM got quite a reputation for doing the same thing, as Mike Smith is about to find out
Yes, it does look a bit FM at first sight, but, according to Tony Hadland (p124 of his excellent book - The Sturmey Archer Story):
"it was the ratchet cut into the LH ball cup that had a tendency to fail, resulting in a low gear pawl bursting through the alloy shell. .........at least three different versions of this ratchet were manufactured, presumably in a bid to overcome the problem. One version used a traditional ramped ratchet; the others were merely slots cut into a cylindrical projection of the ball cup. The slotted versions seem the most suspect."
Steve's diagnosis is different to this, in that the pawl's retaining pin sheared, allowing the pawl to come adrift, get jammed and forced through the shell.
Steve, is the low gear ratchet intact?
Whether Mike Smith experiences this first hand... we'll just have to wait. Perhaps, for a bit of fun, we could run a book?
I couldn't possibly end this post without pointing out that the FM ratios are perhaps the best spaced of all SA hubs - certainly of that era, and it should be no surprise that they are sought after, despite the known reliability problem. The gear change from 2nd to 1st is a little slow - you can feel the clutch engaging when the alignment is correct (as on other four speeds), and the gear cable tension has to be just right to prevent slippages.
I have two FM hubs, a 1953 alloy shelled model with the ramped ratchet and a 1960's steel shelled model with the slotted ratchet. The former is in regular use, the latter a spare - bought on Ebay for a bargain price. An alloy shelled one would have cost a lot more.
Long live the FM - king of hub gears.