Acoustic Research AR4x identification

[Giorgio AR]

Hi, I illustrate this pair of my AR4x, produced at the end of 1969 (as written on the woofer magnets, dated 46 69 and 47 69).
There is a week of age difference between the two woofers, both have a foam ring on the cone (which is very rare, I think I only saw another pair with this ring on the cone).
The serial numbers (which should be close to about 300000) are identified with AR4x Hfa 006195 and Hfa 00750 .. (the last two numbers are illegible)
Why is the numbering not progressive, as usual?
Perhaps this numbering is exclusive for AR4x with ring on the woof

The photos illustrate: the speakers with grilles, speakers without grilles, back of the speakers, the two labels with the model and serial number, the woofer magnets

[ra.ra]

Hi Giorgio, that is a very nice-looking pair of 4x's and you're right, the 8-inch woofers with that damping ring are not very common. By all appearances, those are a fully matching pair - - - both woofers with ring and only four screw holes in basket; red plastic control dials on potentiometer; and identical panel materials for cabinet construction. Grilles, badges, veneer and labels all look virtually identical as well.

There may be no obvious explanation for the jump in serial numbers from ±6000 to ±7000, but perhaps some of these thoughts might offer some context. What you interpret as "fa" is actually a sloppy handwritten version of "FX". AR-4 speaker label serial numbers began with a designation "F", and once the model 4x was introduced this became "FX". Since you are in Europe, and these speakers date from 1969, I am going to suggest that the "H" notation designates the AR facility in Holland, which I believe opened in Amersfoort in 1969...... and it seems comprehensible that the AR printed labels did not yet reflect this Holland address below the usual Cambridge address.

It is also understandable that you would think these s/n's should be much higher by 1969, but it is clear that the Euro numbering did not fit sequentially with USA production units. Just for example, I have several pairs of USA AR-4x's nearby, and their numbers and dates are as follows: ±38000 (1966); ±159000 (1968); ±318000 (1970); and ±383000 (1971). I hope maybe some comments here might be helpful to you. 

[Giorgio AR]

Thank you ra.ra for the answer and for the very understandable explanation: you dated 3 years of construction, it is likely that the production was Dutch.
Apart from my couple, I don't remember seeing AR4x late with low serial numbers, only original early AR4x.