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Could SSSE be Sino-Scottish Szechuan expedition?
At 63 + in the key, it states: "Perianth segments ovate or ovate-oblong, 6--10 mm, at least margin of inner ones irregularly denticulate; filaments usually 1/2--2/3 as long as perianth segments; style shorter than or subequaling ovary." Is the style of your plant longer (A. beesianum) or shorter (A. sikkimense) than the ovary? This might help sway you either way.
Hard to see for sure from the photo, but appears to have blunt tepals with the inner ones only denticulate and the stigma looks to be shorter than the ovary ... this could be the real A. sikkimense?? Apparently it is a variable species with several forms in cultivation.
That's what I was thinking Matt!? Just looked through my old photo's but I'm afraid I don't have any other clearer ones.
Flora Italia (http://luirig.altervista.org/schedeit/ae/allium_coloratum.htm)