I'm not trying or intending to be definitive in my opinion which is partly intuitive...
Everything I've learned about Leerdam (and Maastricht) leads me to assume that there is an exception to every rule..sometimes two!!
OTOH I've been at the museum several times in the last weeks and there are quite a lot of 'mistakes' which have crept into their knowledge base. (and which have been filed with the curator) (I gather that the cabinets are being opened regularly for 'adjustment'

I presume that's a sign of work in progress?! ;-)
http://www.nationaalglasmuseum.nl/collectie/weergave/thema?id=35Like most collectors in this field I feel compelled to see everything verified and documented before I'm really sure it's a fact. (and I've had the glass in my hands, LOL!)
I've started to do my own little bit to clear up some of the errors in the cabinets, but I admit that your particular item is a little late for my main interests so I'm not familiar with the relevant documentary archive. I AM aware that major new 'discoveries' are being made recently in other oeuvres which were previously thought 'fully-documented'.
The museum is currently working hard to complete the online publication of the remaining catalogues, (pop-ups and scripts are a bit dodgy at the moment) and then will be adding the rest of the drawings and photos from it's archive 'in due course', but even then...their records are very far from complete. (partly due to loss/fire damage and partly to secretive collectors, etc.)
Unfortunately it is not easy to identify the source of all the items in the museum depot. If you saw it in the displays then it MAY have come from the factory directly, but may also have been gifted to them with an erroneous description (we have identified several examples of this phenomenon already). Since I am only a bystander at the museum I am restricting my inputs to the fields where I have better knowledge and where original research is still emerging.
Next year there will be several new books which cover 'Undiscovered Leerdam'. Both the Kley Blekxtoon books contained errors and omissions (lack of space was probably also a factor) and some of the designers she just considered 'second rate' and omitted. They include Thomassen who is responsible for many of their best selling designs for functional items in the 60's/70's , and the Heesen family, who made (and make) most and their best work outside the influence of Leerdam at 'de Oude Hoorn'.
http://www.deoudehorn.com/The museum (NGM) has survived periods with little funding and a few dedicated but frustrated volunteers. It has become clear that some labels have been lost and/or moved, that some items have been replaced in the 'wrong' box, etc etc.
The Museum is aware of the many gaps in its knowledge and is hoping that the 'Glass Wiki' which is now linked to the site will be able to start the process of filling them in. Let's pray that technology is on our side now!
As the documents are becoming available then researchers are starting to move in and clear up these matters, so I think you can expect that a book covering the period will be produced before too long,
So
*I don't know for sure myself, I'm just suggesting that you can't entirely remove the question mark on the attribution until you see a signed design drawing.
*'you'll just have to be patient' (argh, as always) and hope that the archive develops and improves with the reopened museum and its determined new curator! The drawing would probably be there and will emerge in due course.
*Many of the items in the depot are being viewed by the public for the first time. Feedback to the exhibits is scattered with all sorts of new and conflicting information which will need to be assessed and verified.
Why am I a bit 'suspicious' of the attribution
a) Never saw this one in the wild before, and I see a LOT of Dutch glass! (and! this looks like a mass market item which I would therefore expect to be familiar even if unknown to me?)
b) The switch from heavy colours (pre war) to lighter and brighter colours post war meant that it was into the '70s before dark-bright colours were produced again at Leerdam. I can't find any other contemporary reference to this pallette in Leerdam around that date.
c) I have a pretty good collection of archives at my fingertips which should cover most items which were retailed from the factory, and I can't see this one! (though there is a big gap in everyone's(?) archive at the date you mention)
d) I know there are still quite a few bugs in the museum's information, so I value their attributions against that background.