Looks like a nice compact construction for such an early lamp with a quartz arc tube.

Nice lamp, but it is not from 1956 unfortunately. It features fat oxide electrodes, confirmed by the "33" in the lamp reference. The ANSI code H33 was introduced specifically for improved 400 W mercury lamps provided with oxide electrodes, which GE introduced in 1958 in the USA. In that company's nomenclature this corresponded to H400xx33-1 from the early-mid-60s forward, GE added the suffix -1 to 33 following the phase out of the original 400W H1 mercury lamps with thoriated electrodes in the early 1960s. If the "Bonus Line" trademark is mentioned in the stamp printed on your lamp, then it was definitely made in 1964, otherwise it is 1972, 1980, or even possibly later. don't be misled by the apparent "early" appearance of the lamp's overall structure, GE was one of the most (if not the most) conservative manufacturer in this regard.

Date Code is 1 2 It has the pre Bonus Line etch . Plus , It has the early sleeve design in blue & white . The sleeve disintegrates if I even look at it wrong . https://www.ebay.com/itm/335693581014 1980 or newer would have the "R" designation for a Mercury lamp .

Nice one anyway, however old it is

No "Bonus Line" and no "R" in the etch, it is 1972 then.

This has the late 50's/early 60's sleeve...the sleeve from 1972 would be blue/yellow/white.

Bold of you to assume that the sleeve has the exact same origin in time and space as the lamp and that nothing happened in 50+ years, like, someone swapping lamp and sleeve... Certainty in the dating of lamps comes from features intrinsic to the lamp, like certain key design elements (oxide electrodes) and the reference printed on the glass bulb (H400A33-1/T16 w/o the Bonus Line brand), because, unlike for the sleeve, which can be swapped, there is a guarantee that those elements associated to the lamp alone haven't changed in time.

On the subject of Tim's earlier remark about the "pre Bonus Line" etch, that's exactly what I meant with the "don't be mislead" remark I made in my first comment here. It may look old but it isn't. A real pre Bonus Line etch (i.e., from before 1958) wouldn't bear the modern H400A33-1/T16 reference, but the old H400-B1 one GE used before the introduction of oxide electrodes. It's this kind of details which really matters when determining the actual age of a lamp.

So, what the e-bay listing shows for sure is a modern 400 W mercury lamp with oxide electrodes, combined with an older paper sleeve from the late 50's/early 60's. It's not the first time a lamp came from such market (ebay) with the wrong sleeve, it happened to me a few times and for a variety of reasons.

I've certainly come across lamps and sleeves that don't match.

The worst I've seen are dead or badly worn-out lamps combined with pristine sleeves, and the lot being sold as NOS. Often, sellers are totally oblivious to the real state of the lamp as they lack knowledge (or good judgement) in that area, and they truly believe they are selling perfectly unused items... you really have to pay attention to details when buying stuff from those marketplaces, especially historical items which have been passed along in the course of many decades.