I found an eBay
listing for pane 1510d that shows the gum side of the stamps. They do appear to be dull gum. Since there is no mention of gum in Scott for any panes, I would assume that all 1510 panes (1510b in BK124, 1510c in BK125 & 1510d in BK126) had dull gum. This is new to me … I had always assumed that Scott included gum type whenever any entry in a listing involved multiple gum types. For issues involving both sheet & booklet panes, that is apparently true only for each format, sheet or booklet … not for the two formats together.
So I think eyeonwall's memory is far better than mine. ALL 1510 booklet panes must have had dull gum.Added
: Just discovered that the seller, in his description, indicates that the stamps do have dull gum. Item # 143357171852
Also located a 1510c pane that a seller describes as having dull gum.