This is a 3H2686 bakelite spacer for a 1 1/8 inch SU commonly used as twins on a frogeye sprite.  It is true imperial at 9/16" thick or about 14.3 mm.

Note it is a more like an old-school bakelite, not the more modern phenolics, so treat it in a careful bakelite way.

Note there is now a high quality gasket kit AUE800.  There are also cheapo low quality 3h2687 $2 1 1/8 gaskets as well.

There are only two held in stock, due to the relatively high price... sorry ... yes the price is correct.... it costs what it costs...   The listing is for ONE only. I repeat ONE ONLY. To order two, change the quantity box to two. The postage will combine automatically.

Also..... because of the cost, and because of the waiting period to re-order replacement ones in, and another reason....    I only use red bag or courier post, both fully signed-for, not cheaper letter post.  This postage stays the same for more than one.