Maybe I can shed some light - my subreddit (r/ModelM) ran a QnA with Unicomp and one of the questions I asked is why their M4/M4-1s were retired. Their response is that the Mighty Mouse (their marketing name for it) was a recreation of the original Lexmark design for a client that backed out at the last minute, thus they had to sell their stock.
QnA source:
https://www.reddit.com/r/modelm/wiki/unicomp_qaIBM also seemingly ordered from some of that stock since I have a 1999 IBM by Unicomp M4-1. One of the main markets for M4-1s seemed to be ThinkPads, as many hardware maintenance manuals from the mid-'90s mentioned them. This continued beyond 1996, something you can test by looking at the ThinkPad 380Z HMM from August 1998. So it seems Unicomp were also actively making new ones for the IBM (M4/M4-1s were never made at Greenock, AFAIK) and it's probably where my one came from. Considering a few early ThinkPads also used a similar sleeves mechanism, I imagine they were geared towards being an option if you're connecting a ThinkPad to a docking station or some other desktop setting.
unicomp's rubber domes are quite different from IBM rubber domes
The numeric keypad attachment I have for it is an original M4-1 (1995 IBM by Lexmark) and I can confirm it's buckling rubber sleeves (not simple domes) feel exactly the same between them. The rubber cylinder is just a tactile element and not involved in the contact mechanism - the keycaps themselves 'stabs' the membrane directly with no mush feeling at all. It seems Unicomp faithfully recreated the design, and I'm glad, since it's one of my favourite tactile-only switches.
If you're curious, I have more details on the keypad and the sleeves on my article about it:
https://sharktastica.co.uk/articles/keypad_m41.php.
So they did not simply put a laptop keyboard in a case but created something with more travel.
Technically, it actually is, but from a different era of laptops. The original M4 (non-TrackPoint variant) was just a desktop adaptation of the Model M3 keyboard found on the IBM PS/2 L40SX machine and its sidecar numeric keypad.
(Photo credits: Wikipedia)
The layout is a dead giveaway (excuse the ANSI/ISO different of course).