Patek Philippe | Reference 3971, A yellow gold perpetual calendar chronograph wristwatch with moon phases, 24 hours, leap year indication and snap on sapphire crystal display back, Made in 1988 | 百達翡麗 | 型號3971 黃金萬年曆計時腕錶，備月相、24小時、閏年顯示及藍寶石水晶底蓋，1988年製

The reference 3970 was first introduced in 1986 and was a successor to the celebrated 2499. Patek reduced the case diameter of the 3970, making it 1.5 mm smaller than the 2499. The reference was discontinued in 2004. All case metals were available and as the reference evolved, 4 different series can be identified. Within the first year of production, there was a sister reference with sapphire case back. The present watch is from this uber rare family and is listed as a 3971. What further elevates this example is that the sapphire back is snapped on to the case. Less than 20 examples of the snap on case back 3971 are known, making it the rarest variant of all. This wristwatch has good proportions and strong fluted lugs with hallmarks under the lugs and a 18k stamp indicating the watch was made for the American market.