I got a pair of Shimano SH-IC100M indoor cycling shoes because they're the only ones that come in EU 49. They have 5 captive nuts to fit either SPD-SL (3 bolt) or SPD (2 bolt). They're listed as compatible with either the SM-SH40 or SM-SH41 adapter.

The SM-SH40 adapter just uses the 2 SPD holes with longer bolts (14 millimeters) through a shim-with-wings (a "pontoon") that goes around the cleat to make it "walkable." It seems to be discontinued.

The SM-SH41 adapter (instructions) comes with a plate with 3 holes and 2 nuts. There are 3 bolts to go through the plate into a SPD-SL shoe and it comes with 2 bolts for the SPD cleat & a "pontoon." It has no need for the extra two nuts on my shoes.

The 5-bolt shoe and SM-SH40 adapter seemed like such a reasonable compromise. Why kill it? I guess this means the SH-IC100M shoes will be revised at some point to only have 3 nuts.

Here's the crazy thing: the instructions for the SM-SH41 adapter say "Note that you must use the special SM-SH40 cleat mounting bolts." Whaaa? I'll measure the bolts that come with it & update this post when they arrive. I hope this is just an error of copy-and-paste-without-proofreading.

It is nice that if I ever get a Peloton & don't want to change pedals, I could mount Peloton's cleats to these shoes.

(I'm aware these little things are machine screws, not bolts)

Update:
The screws that came with the SM-SH56 cleats were 11.4mm long.
The screws for the 2-bolt cleats that came with the SM-SH41 adapters were 9.3mm long.

These lengths include the head. The heads, which are flat-head with beveled, but curved in a bit sides, are 1.7 mm of that overall length.

I did find reviews of the discontinued SM-SH40 that said durability of the pontoons was poor, so maybe that, combined with the hassle of aligning all 5 bolts, helped kill it.

I use the Shimano SH-RP100 SL (SH RP1) size 48 wide (larger sizes available). Not the highest quality cycling shoe, but it comes in wide widths and that’s mostly what I am looking for. They have a 3-bolt/screw pattern to accommodate the cleats you want to use. No adapter necessary.

FYI, the “Peloton cleats” are actually LOOK Delta cleats. Those are what I’ve used since the 90s. They were among the earlier clipless formats. For whatever reason, LOOK discontinued them in the early 2000s, and switched to their current NEO format (not interchangeable). As luck would have it, I keep +/- eight bikes going all with Delta format pedals. I was worried that the Delta cleats would become difficult to obtain, and/or I’d have to switch out the pedals on all my bikes. Fortunately, that’s when the spinning craze was taking off and they adopted the LOOK Delta format for their pedals.

Dan

Maybe Shimano went away from the SH40 because it kinda requires that the three holes and the two holes are aligned in a particular way with each other, and some shoes might not have the right alignment, causing problems for customers. The SH41 sticks with a just one of those sets of holes, avoiding that issue.

I recall seeing - many years ago - a 2 bolt MTB cleat that had a pair of short pontoons surrounding it, mounted on a thin plate.

Edit. These: