First of all thank you for accepting me to the forum.
As far as I understand I-16 Type 4s, and first series of Type 5s prior to armor upgrades had seats with narrow backrest. Oddly, the restored I-16 which is now on display at Military Aviation Museum in Virginia Beach has this early type of the seat installed. See the photo found on net-maquettes.com walkaround:
But I guess that originally this seat should have leather upholstery attached to the backrest. Has anyone any photo or drawing showing such seat with padding in place? I suppose it would be something similar to upholstery still preserved in UTI-4 displayed in Finland. See the photo uploaded to Britmodeller forum: