I got my Mk7 r.d. to work and I have been enjoying it. With about 10mm of spacers, I was able to install the claw mount in a position that would allow me to reach the smallest (14T) cog. The chain wrap appears to be what is recommended. Sometimes, the derailleur won't immediately go into the 28T low gear, so I bring it out, back the the 24T, and then if I try again, it will usually drop in.
I took two links out of a 116 link chain, so it is 114, I am pretty sure. The chain is very slack in the 14T cog setting, but I would need a longer B tension screw to bring the body of the derailleur back to take up that slack. Maybe I should have left the chain longer. I'm not sure.
I'm not really sure what role the spring tension plays, but I have it set pretty high. I'm getting all four gears, including 28T, while I am using the 46T (inner) chainring.
I did not know that this type of crankset can not be used with a front derailleur, so I'll be replacing it with a later Williams.

Subscribing! What is the problem with the crank set?

That looks like the compact I use with my Campy Gran Sport, no issues.

It's hard to say, from the photo, but perhaps the front derailleur is mounted a little too high.

The need for spacers indicates the rear derailleur is bent. I don't think I've ever seen one of these (cyclo Benelux, simplex tour de France, etc) that wasn't bent, so don't be alarmed. If the bike falls over on the right side, the derailleur ends up holding the rear of the bike, and it bends. Parallelogram derailleurs, in contrast, deflect (as if you were downshifting) in the same circumstances, so are less likely to get bent.

If you can confirm that the derailleur is indeed bent, and you want to straighten it, alas you have to disassemble it almost completely.

The crank arm is far too close to the outer chainring to fit a cage in between. Ebay member Velomadison informs us in his current offering description that:

"These are not designed for a front derailleur, rather to be set up with
one or the other for different desired gearing, often with a rear wheel
with butterfly nuts."

There is 1-2mm of space betwen the crank arm and the rivet(s)..

It probably is a bit too high. Since it is not compatible with this crank, I'm not worrying about its position right away.
If anything, it appears that the rear derailleur body is pointing starboard, but the leaf spring tube may be a pointing a bit forward. I've had it completely apart, so it's not really a problem assembling/disassembling it. I'll look at photos of some other examples, preferably N.O.S. and see what I can determine.
I'd just like to reel in some of the slack when the chain is on the small cog, but it doesn't skip and I might be able to manage this way. Overall, I'm pleased with its performance already, even now.

It depends on which of the Cyclo Benelux Mark 7's you have. They came in 3sp 1/8", 4sp 1/8". 4 sp 3/32" and 5 speed 3/32".
the sprocket range was 13T-24T and with a double chainwheel total capacity was 15T.
The RD was not designed for the capacity you are expecting of it.
see Cyclo - Benelux Mark 7 Gear scan 2

All true. My friend ran a 1/8" 3-speed 13-19-25 cogset on a Sturmey-Archer AW hub, and the bandspring rear derailleur worked like cr@p with it. I did not have much better luck with my 1/8" 4-speed 14-16-18-20 cogset on a S-A hub until I switched to a REAL derailleur (first-generation Campag. GS), which worked like a champ.

Benelux Tweaking.

At first I thought this might be a thread on where to score meth in the western European low countries.

My derailleur model will accept both 5- and 4-speed freewheels, although it recommends a 1/8" wide chain for 4-speed and 3/32" for 5-speed. I have to put more miles on it to see whether the r.d. performs reliably with the 14-19-24-28 block, but so-far, so-good. If I have to fidget and fuss a bit, it is worth it to me on this project, to be able to keep the bike as "period" and as British as possible.
If I enounter insurmountable problems, I'll search in earnest for something period, but with a more up-to-date design, like the Simplex RaidExport 32 2nd version. That particular early-60's derailleur is rated for 32T.

Ah, I see. Thank you.