That same I was ready to add... The 31a was designed with the thick tonearm and the 5a/5b soundboxes. There shouldn't be any mismatch in impedances; the 102/104/130/145/150 gramophones had sound systems that I'd say were shorter than the 31a, with short tonearm (as 102/104) but internal conduit and this long wide horn. The 5a/5b sound well with the shortest 102 system, as far as with the longest and largest ones (158/202)...
The European model 32 was designed pretty similar, but using the narrow swan neck tonearm, the 4 soundbox and ---i imagine--- a narrower conduit, leading to the same horn. BTW, the narrow swan neck tonearm, even in the smallest version (101 and tabletop models 103/109/126/127) was noticeably longer than the short fat swan neck tonearm of the 102/104. I suppose the larger machines with narrow tonearm (consoles and uprights of that era) used longer narrow tonearms. At least the one in the 461/511 seemed longer.
The fast tonearm was short in the 102/104, but maybe the tabletops 130/150 plus the small 145 upright and the consoles and uprights (from 157 up the scale} used longer sizes (an intermediate length exists for these...??). The 193/4 and the 202/3 used the largest size, a pretty long tonearm with very good tracking alignment.
The end: the 5/5a/5b soundboxes are used in all these systems, from the short 102 to the largest 202. The same can be said of the no4 soundbox, used in all machines of the narrow tonearm generation, the shortest being the 101 and the longest probably the 211 or the saxophone horn models.
Aside of that, I'm not sure if the system of the 101 is so short. Yesterday I was at an afternoon private listening party with myself, using the 101 and playing piano records (Arthur Schnabel jr. recording of the Mendelssongs without words). And the sound was incredibly good. It was in my kitchen/office, at the after meal, just the dishes and ancillaries removed, and the machine and records on the tablecloth. While taking a coffee... This little marvel of 101 filled the room with warm sound and beautiful music...! Terrible! The open lid acting as reflector actually elongates the actual sound system length further than its material 3-feet conduit.
The 102, having a slightly shorter sound conduit, also sounds well (actually better). But this is a different design, starting at a wider bore, and ending also at a wider horn mouth. The lid reflector also plays its part. The power of the 5-type soundbox as well as its frequency content is also greater than the mica type. Interesting...
--
That said, with well restored examples of both systems (the no4 based and the no5/a/b) the sound is very good. The treble and bass is better in the later system, mostly noticeably in the treble register. But many collectors have expressed the impossibility of choosing one of them alone, both systems having their own charms. The narrow one has a mellowness which the wider one (even regarded as an upgrade, as it was intended) cannot match...
-- latest edit, i swear...

---
Each time I decide to have a session with the 101, I'm mostly amazed at the good sound of this little machine (if the room size is not very wide). And I always put myself on the verge of deciding which machine alone I would keep, in case a decision had to be taken (kind of desert island affair...) And I'm always tempted to say I would choose the 101... it's cheap, pretty common and easy to buy even 88 years after its formal demise, there are plenty of spare parts, it's easy to maintain, and the sound is GREAT. For me, the 101 is the best overall achievement of The Gramophone Co. How many they made? Provided that today or is actually very easy to find one, in every corner of the world...
