Atco Deluxe 17

The original chassis ID plate would have been either a round disc or a brass tag held onto the right hand ( viewed from the operator’s position) chassis side plate by one of the hex head set screws holding either the cutter unit or rear roller bearing in place. 

If the disc or tag have disappeared, accurate dating will be almost impossible.

Difficult to see from your images but it looks as though the thrust pads on the main clutch yokes are completely worn out. One early machines they were a fibre material and later a hard plastic. In both cases held to the yokes by a bifurcated rivet.

E12265 is the cylinder block number of the 98cc model 3 engine. This was the final model of the cast iron 98cc and had a valve chest breather unit attached using 2 set screws instead of a cover plate with a nut on a central stud of the older ones.    

This change was noted in 1977.

By 1979? or at least 1980? the aluminium A98 engine was being used instead.

 The basic chassis design, with detail changes over time, was used from late 1960s up to change to the new model, Atco Commodore, in 1983.

Early chassis series had BSF threads, the newer series had UNF.  I always think this change happened when aluminium engine was used but not fully sure.

I feel that with having 2 front roller height adjuster levers that the chassis looks about right for the possible engine age?....although swaps can happen over years of use.

   C.

To avoid confusion it should be noted that the E12265 was the foundry casting number for the 98cc block and not the part number used for ordering replacement parts. That number for the late type block with the two screw breather was A1881 which Atco referred to as a "stock number" and was later superceded to an L prefix number which they called a "computer number". As far as dating goes, Atco were somewhat unhelpful in not always dating manuals and bulletins. A user manual that I have showing the "two screw" breather does have a publication number of RS.5460/4/76 which could possibly fit with April 1976 as being the approximate introduction date for that style of block. 

As far as chassis dating goes it does appear that by that time they may have dropped the metal tag ID discs and tags and moved over to less durable adhesive tin foil "rating labels". Another user manual bearing a publication number of "L21129 Temporary", has a statement on the first page saying "Before ordering spares,  check the rating label for a letter U indicating Unified Threads”. However it still refers to the cast iron engines which throws doubt on the thread change being linked to the adoption of the A98 engine.